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<CENTER><A HREF = "Section_commands.html">Previous Section</A> - <A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A> - <A HREF = "Section_example.html">Next Section</A>
</CENTER>
<HR>
<H3>4. How-to discussions
</H3>
<P>The following sections describe what commands can be used to perform
certain kinds of LAMMPS simulations.
</P>
4.1 <A HREF = "#4_1">Restarting a simulation</A><BR>
4.2 <A HREF = "#4_2">2d simulations</A><BR>
4.3 <A HREF = "#4_3">CHARMM and AMBER force fields</A><BR>
4.4 <A HREF = "#4_4">Running multiple simulations from one input script</A><BR>
4.5 <A HREF = "#4_5">Parallel tempering</A><BR>
4.6 <A HREF = "#4_6">Granular models</A><BR>
4.7 <A HREF = "#4_7">TIP3P water model</A><BR>
4.8 <A HREF = "#4_8">TIP4P water model</A><BR>
4.9 <A HREF = "#4_9">SPC water model</A><BR>
4.10 <A HREF = "#4_10">Coupling LAMMPS to other codes</A><BR>
4.11 <A HREF = "#4_11">Visualizing LAMMPS snapshots</A><BR>
4.12 <A HREF = "#4_12">Non-orthogonal simulation boxes</A><BR>
4.13 <A HREF = "#4_13">NEMD simulations</A><BR>
4.14 <A HREF = "#4_14">Extended spherical and aspherical particles</A><BR>
4.15 <A HREF = "#4_15">Output from LAMMPS</A><BR>
4.16 <A HREF = "#4_16">Thermostatting, barostatting and computing temperature</A> <BR>
<P>The example input scripts included in the LAMMPS distribution and
highlighted in <A HREF = "Section_example.html">this section</A> also show how to
setup and run various kinds of problems.
</P>
<HR>
<A NAME = "4_1"></A><H4>4.1 Restarting a simulation
</H4>
<P>There are 3 ways to continue a long LAMMPS simulation. Multiple
<A HREF = "run.html">run</A> commands can be used in the same input script. Each
run will continue from where the previous run left off. Or binary
restart files can be saved to disk using the <A HREF = "restart.html">restart</A>
command. At a later time, these binary files can be read via a
<A HREF = "read_restart.html">read_restart</A> command in a new script. Or they can
be converted to text data files and read by a
<A HREF = "read_data.html">read_data</A> command in a new script. <A HREF = "Section_tools.html">This
section</A> discusses the <I>restart2data</I> tool that is
used to perform the conversion.
</P>
<P>Here we give examples of 2 scripts that read either a binary restart
file or a converted data file and then issue a new run command to
continue where the previous run left off. They illustrate what
settings must be made in the new script. Details are discussed in the
documentation for the <A HREF = "read_restart.html">read_restart</A> and
<A HREF = "read_data.html">read_data</A> commands.
</P>
<P>Look at the <I>in.chain</I> input script provided in the <I>bench</I> directory
of the LAMMPS distribution to see the original script that these 2
scripts are based on. If that script had the line
</P>
<PRE>restart 50 tmp.restart
</PRE>
<P>added to it, it would produce 2 binary restart files (tmp.restart.50
and tmp.restart.100) as it ran.
</P>
<P>This script could be used to read the 1st restart file and re-run the
last 50 timesteps:
</P>
<PRE>read_restart tmp.restart.50
</PRE>
<PRE>neighbor 0.4 bin
neigh_modify every 1 delay 1
</PRE>
<PRE>fix 1 all nve
fix 2 all langevin 1.0 1.0 10.0 904297
</PRE>
<PRE>timestep 0.012
</PRE>
<PRE>run 50
</PRE>
<P>Note that the following commands do not need to be repeated because
their settings are included in the restart file: <I>units, atom_style,
special_bonds, pair_style, bond_style</I>. However these commands do
need to be used, since their settings are not in the restart file:
<I>neighbor, fix, timestep</I>.
</P>
<P>If you actually use this script to perform a restarted run, you will
notice that the thermodynamic data match at step 50 (if you also put a
"thermo 50" command in the original script), but do not match at step
100. This is because the <A HREF = "fix_langevin.html">fix langevin</A> command
uses random numbers in a way that does not allow for perfect restarts.
</P>
<P>As an alternate approach, the restart file could be converted to a data
file using this tool:
</P>
<PRE>restart2data tmp.restart.50 tmp.restart.data
</PRE>
<P>Then, this script could be used to re-run the last 50 steps:
</P>
<PRE>units lj
atom_style bond
pair_style lj/cut 1.12
pair_modify shift yes
bond_style fene
special_bonds 0.0 1.0 1.0
</PRE>
<PRE>read_data tmp.restart.data
</PRE>
<PRE>neighbor 0.4 bin
neigh_modify every 1 delay 1
</PRE>
<PRE>fix 1 all nve
fix 2 all langevin 1.0 1.0 10.0 904297
</PRE>
<PRE>timestep 0.012
</PRE>
<PRE>reset_timestep 50
run 50
</PRE>
<P>Note that nearly all the settings specified in the original <I>in.chain</I>
script must be repeated, except the <I>pair_coeff</I> and <I>bond_coeff</I>
commands since the new data file lists the force field coefficients.
Also, the <A HREF = "reset_timestep.html">reset_timestep</A> command is used to tell
LAMMPS the current timestep. This value is stored in restart files,
but not in data files.
</P>
<HR>
<A NAME = "4_2"></A><H4>4.2 2d simulations
</H4>
<P>Use the <A HREF = "dimension.html">dimension</A> command to specify a 2d simulation.
</P>
<P>Make the simulation box periodic in z via the <A HREF = "boundary.html">boundary</A>
command. This is the default.
</P>
<P>If using the <A HREF = "create_box.html">create box</A> command to define a
simulation box, set the z dimensions narrow, but finite, so that the
create_atoms command will tile the 3d simulation box with a single z
plane of atoms - e.g.
</P>
<PRE><A HREF = "create_box.html">create box</A> 1 -10 10 -10 10 -0.25 0.25
</PRE>
<P>If using the <A HREF = "read_data.html">read data</A> command to read in a file of
atom coordinates, set the "zlo zhi" values to be finite but narrow,
similar to the create_box command settings just described. For each
atom in the file, assign a z coordinate so it falls inside the
z-boundaries of the box - e.g. 0.0.
</P>
<P>Use the <A HREF = "fix_enforce2d.html">fix enforce2d</A> command as the last
defined fix to insure that the z-components of velocities and forces
are zeroed out every timestep. The reason to make it the last fix is
so that any forces induced by other fixes will be zeroed out.
</P>
<P>Many of the example input scripts included in the LAMMPS distribution
are for 2d models.
</P>
<P>IMPORTANT NOTE: Some models in LAMMPS treat particles as extended
spheres, as opposed to point particles. In 2d, the particles will
still be spheres, not disks, meaning their moment of inertia will be
the same as in 3d.
</P>
<HR>
<A NAME = "4_3"></A><H4>4.3 CHARMM and AMBER force fields
</H4>
<P>There are many different ways to compute forces in the <A HREF = "http://www.scripps.edu/brooks">CHARMM</A>
and <A HREF = "http://amber.scripps.edu">AMBER</A> molecular dynamics codes, only some of which are
available as options in LAMMPS. A force field has 2 parts: the
formulas that define it and the coefficients used for a particular
system. Here we only discuss formulas implemented in LAMMPS. Setting
coefficients is done in the input data file via the
<A HREF = "read_data.html">read_data</A> command or in the input script with
commands like <A HREF = "pair_coeff.html">pair_coeff</A> or
<A HREF = "bond_coeff.html">bond_coeff</A>. See <A HREF = "Section_tools.html">this section</A> for
additional tools that can use CHARMM or AMBER to assign force field
coefficients and convert their output into LAMMPS input.
</P>
<P>See <A HREF = "#MacKerell">(MacKerell)</A> for a description of the CHARMM force
field. See <A HREF = "#Cornell">(Cornell)</A> for a description of the AMBER force
field.
</P>
<P>These style choices compute force field formulas that are consistent
with common options in CHARMM or AMBER. See each command's
documentation for the formula it computes.
</P>
<UL><LI><A HREF = "bond_style.html">bond_style</A> harmonic
<LI><A HREF = "angle_style.html">angle_style</A> charmm
<LI><A HREF = "dihedral_style.html">dihedral_style</A> charmm
<LI><A HREF = "pair_style.html">pair_style</A> lj/charmm/coul/charmm
<LI><A HREF = "pair_style.html">pair_style</A> lj/charmm/coul/charmm/implicit
<LI><A HREF = "pair_style.html">pair_style</A> lj/charmm/coul/long
</UL>
<UL><LI><A HREF = "special_bonds.html">special_bonds</A> charmm
<LI><A HREF = "special_bonds.html">special_bonds</A> amber
</UL>
<HR>
<A NAME = "4_4"></A><H4>4.4 Running multiple simulations from one input script
</H4>
<P>This can be done in several ways. See the documentation for
individual commands for more details on how these examples work.
</P>
<P>If "multiple simulations" means continue a previous simulation for
more timesteps, then you simply use the <A HREF = "run.html">run</A> command
multiple times. For example, this script
</P>
<PRE>units lj
atom_style atomic
read_data data.lj
run 10000
run 10000
run 10000
run 10000
run 10000
</PRE>
<P>would run 5 successive simulations of the same system for a total of
50,000 timesteps.
</P>
<P>If you wish to run totally different simulations, one after the other,
the <A HREF = "clear.html">clear</A> command can be used in between them to
re-initialize LAMMPS. For example, this script
</P>
<PRE>units lj
atom_style atomic
read_data data.lj
run 10000
clear
units lj
atom_style atomic
read_data data.lj.new
run 10000
</PRE>
<P>would run 2 independent simulations, one after the other.
</P>
<P>For large numbers of independent simulations, you can use
<A HREF = "variable.html">variables</A> and the <A HREF = "next.html">next</A> and
<A HREF = "jump.html">jump</A> commands to loop over the same input script
multiple times with different settings. For example, this
script, named in.polymer
</P>
<PRE>variable d index run1 run2 run3 run4 run5 run6 run7 run8
shell cd $d
read_data data.polymer
run 10000
shell cd ..
clear
next d
jump in.polymer
</PRE>
<P>would run 8 simulations in different directories, using a data.polymer
file in each directory. The same concept could be used to run the
same system at 8 different temperatures, using a temperature variable
and storing the output in different log and dump files, for example
</P>
<PRE>variable a loop 8
variable t index 0.8 0.85 0.9 0.95 1.0 1.05 1.1 1.15
log log.$a
read data.polymer
velocity all create $t 352839
fix 1 all nvt $t $t 100.0
dump 1 all atom 1000 dump.$a
run 100000
next t
next a
jump in.polymer
</PRE>
<P>All of the above examples work whether you are running on 1 or
multiple processors, but assumed you are running LAMMPS on a single
partition of processors. LAMMPS can be run on multiple partitions via
the "-partition" command-line switch as described in <A HREF = "Section_start.html#2_6">this
section</A> of the manual.
</P>
<P>In the last 2 examples, if LAMMPS were run on 3 partitions, the same
scripts could be used if the "index" and "loop" variables were
replaced with <I>universe</I>-style variables, as described in the
<A HREF = "variable.html">variable</A> command. Also, the "next t" and "next a"
commands would need to be replaced with a single "next a t" command.
With these modifications, the 8 simulations of each script would run
on the 3 partitions one after the other until all were finished.
Initially, 3 simulations would be started simultaneously, one on each
partition. When one finished, that partition would then start
the 4th simulation, and so forth, until all 8 were completed.
</P>
<HR>
<A NAME = "4_5"></A><H4>4.5 Parallel tempering
</H4>
<P>The <A HREF = "temper.html">temper</A> command can be used to perform a parallel
tempering or replica-exchange simulation where multiple copies of a
simulation are run at different temperatures on different sets of
processors, and Monte Carlo temperature swaps are performed between
pairs of copies.
</P>
<P>Use the -procs and -in <A HREF = "Section_start.html#2_6">command-line switches</A>
to launch LAMMPS on multiple partitions.
</P>
<P>In your input script, define a set of temperatures, one for each
processor partition, using the <A HREF = "variable.html">variable</A> command:
</P>
<PRE>variable t world 300.0 310.0 320.0 330.0
</PRE>
<P>Define a fix of style <A HREF = "fix_nvt.html">nvt</A> or <A HREF = "fix_langevin.html">langevin</A>
to control the temperature of each simulation:
</P>
<PRE>fix myfix all nvt $t $t 100.0
</PRE>
<P>Use the <A HREF = "temper.html">temper</A> command in place of a <A HREF = "run.html">run</A>
command to perform a simulation where tempering exchanges will take
place:
</P>
<PRE>temper 100000 100 $t myfix 3847 58382
</PRE>
<HR>
<A NAME = "4_6"></A><H4>4.6 Granular models
</H4>
<P>Granular system are composed of spherical particles with a diameter,
as opposed to point particles. This means they have an angular
velocity and torque can be imparted to them to cause them to rotate.
</P>
<P>To run a simulation of a granular model, you will want to use
the following commands:
</P>
<UL><LI><A HREF = "atom_style.html">atom_style</A> granular
<LI><A HREF = "fix_nve_sphere.html">fix nve/sphere</A>
<LI><A HREF = "fix_gravity.html">fix gravity</A>
</UL>
<P>This compute
</P>
<UL><LI><A HREF = "compute_erotate_sphere.html">compute erotate/sphere</A>
</UL>
<P>calculates rotational kinetic energy which can be <A HREF = "Section_howto.html#4_15">output with
thermodynamic info</A>.
</P>
<P>Use one of these 3 pair potentials, which compute forces and torques
between interacting pairs of particles:
</P>
<UL><LI><A HREF = "pair_style.html">pair_style</A> gran/history
<LI><A HREF = "pair_style.html">pair_style</A> gran/no_history
<LI><A HREF = "pair_style.html">pair_style</A> gran/hertzian
</UL>
<P>These commands implement fix options specific to granular systems:
</P>
<UL><LI><A HREF = "fix_freeze.html">fix freeze</A>
<LI><A HREF = "fix_pour.html">fix pour</A>
<LI><A HREF = "fix_viscous.html">fix viscous</A>
<LI><A HREF = "fix_wall_gran.html">fix wall/gran</A>
</UL>
<P>The fix style <I>freeze</I> zeroes both the force and torque of frozen
atoms, and should be used for granular system instead of the fix style
<I>setforce</I>.
</P>
<P>For computational efficiency, you can eliminate needless pairwise
computations between frozen atoms by using this command:
</P>
<UL><LI><A HREF = "neigh_modify.html">neigh_modify</A> exclude
</UL>
<HR>
<A NAME = "4_7"></A><H4>4.7 TIP3P water model
</H4>
<P>The TIP3P water model as implemented in CHARMM
<A HREF = "#MacKerell">(MacKerell)</A> specifies a 3-site rigid water molecule with
charges and Lennard-Jones parameters assigned to each of the 3 atoms.
In LAMMPS the <A HREF = "fix_shake.html">fix shake</A> command can be used to hold
the two O-H bonds and the H-O-H angle rigid. A bond style of
<I>harmonic</I> and an angle style of <I>harmonic</I> or <I>charmm</I> should also be
used.
</P>
<P>These are the additional parameters (in real units) to set for O and H
atoms and the water molecule to run a rigid TIP3P-CHARMM model with a
cutoff. The K values can be used if a flexible TIP3P model (without
fix shake) is desired. If the LJ epsilon and sigma for HH and OH are
set to 0.0, it corresponds to the original 1983 TIP3P model
<A HREF = "#Jorgensen">(Jorgensen)</A>.
</P>
<P>O mass = 15.9994<BR>
H mass = 1.008 <BR>
</P>
<P>O charge = -0.834<BR>
H charge = 0.417 <BR>
</P>
<P>LJ epsilon of OO = 0.1521<BR>
LJ sigma of OO = 3.1507<BR>
LJ epsilon of HH = 0.0460<BR>
LJ sigma of HH = 0.4000<BR>
LJ epsilon of OH = 0.0836<BR>
LJ sigma of OH = 1.7753 <BR>
</P>
<P>K of OH bond = 450<BR>
r0 of OH bond = 0.9572 <BR>
</P>
<P>K of HOH angle = 55<BR>
theta of HOH angle = 104.52 <BR>
</P>
<P>These are the parameters to use for TIP3P with a long-range Coulombic
solver (Ewald or PPPM in LAMMPS):
</P>
<P>O mass = 15.9994<BR>
H mass = 1.008 <BR>
</P>
<P>O charge = -0.830<BR>
H charge = 0.415 <BR>
</P>
<P>LJ epsilon of OO = 0.102<BR>
LJ sigma of OO = 3.188<BR>
LJ epsilon, sigma of OH, HH = 0.0 <BR>
</P>
<P>K of OH bond = 450<BR>
r0 of OH bond = 0.9572 <BR>
</P>
<P>K of HOH angle = 55<BR>
theta of HOH angle = 104.52 <BR>
</P>
<HR>
<A NAME = "4_8"></A><H4>4.8 TIP4P water model
</H4>
<P>The four-point TIP4P rigid water model extends the traditional
three-point TIP3P model by adding an additional site, usually
massless, where the charge associated with the oxygen atom is placed.
This site M is located at a fixed distance away from the oxygen along
the bisector of the HOH bond angle. A bond style of <I>harmonic</I> and an
angle style of <I>harmonic</I> or <I>charmm</I> should also be used.
</P>
<P>Currently, only a four-point model for long-range Coulombics is
implemented via the LAMMPS <A HREF = "pair_lj.html">pair style
lj/cut/coul/long/tip4p</A>. A cutoff version may be added
the future. For both models, the bond lengths and bond angles should
be held fixed using the <A HREF = "fix_shake.html">fix shake</A> command.
</P>
<P>These are the additional parameters (in real units) to set for O and H
atoms and the water molecule to run a rigid TIP4P model with a cutoff
<A HREF = "#Jorgensen">(Jorgensen)</A>. Note that the OM distance is specified in
the <A HREF = "pair_style.html">pair_style</A> command, not as part of the pair
coefficients.
</P>
<P>O mass = 15.9994<BR>
H mass = 1.008 <BR>
</P>
<P>O charge = -1.040<BR>
H charge = 0.520 <BR>
</P>
<P>r0 of OH bond = 0.9572<BR>
theta of HOH angle = 104.52 <BR>
</P>
<P>OM distance = 0.15 <BR>
</P>
<P>LJ epsilon of O-O = 0.1550<BR>
LJ sigma of O-O = 3.1536<BR>
LJ epsilon, sigma of OH, HH = 0.0 <BR>
</P>
<P>These are the parameters to use for TIP4P with a long-range Coulombic
solver (Ewald or PPPM in LAMMPS):
</P>
<P>O mass = 15.9994<BR>
H mass = 1.008 <BR>
</P>
<P>O charge = -1.0484<BR>
H charge = 0.5242 <BR>
</P>
<P>r0 of OH bond = 0.9572<BR>
theta of HOH angle = 104.52 <BR>
</P>
<P>OM distance = 0.1250 <BR>
</P>
<P>LJ epsilon of O-O = 0.16275<BR>
LJ sigma of O-O = 3.16435<BR>
LJ epsilon, sigma of OH, HH = 0.0 <BR>
</P>
<HR>
<A NAME = "4_9"></A><H4>4.9 SPC water model
</H4>
<P>The SPC water model specifies a 3-site rigid water molecule with
charges and Lennard-Jones parameters assigned to each of the 3 atoms.
In LAMMPS the <A HREF = "fix_shake.html">fix shake</A> command can be used to hold
the two O-H bonds and the H-O-H angle rigid. A bond style of
<I>harmonic</I> and an angle style of <I>harmonic</I> or <I>charmm</I> should also be
used.
</P>
<P>These are the additional parameters (in real units) to set for O and H
atoms and the water molecule to run a rigid SPC model with long-range
Coulombics (Ewald or PPPM in LAMMPS).
</P>
<P>O mass = 15.9994<BR>
H mass = 1.008 <BR>
</P>
<P>O charge = -0.820<BR>
H charge = 0.410 <BR>
</P>
<P>LJ epsilon of OO = 0.1553<BR>
LJ sigma of OO = 3.166<BR>
LJ epsilon, sigma of OH, HH = 0.0 <BR>
</P>
<P>r0 of OH bond = 1.0<BR>
theta of HOH angle = 109.47 <BR>
</P>
<P>To use SPC with a long-range Coulombic solver (Ewald or PPPM in
LAMMPS), the only parameters that change are the partial charge
assignments:
</P>
<P>O charge = -0.8476<BR>
H charge = 0.4238 <BR>
</P>
<HR>
<A NAME = "4_10"></A><H4>4.10 Coupling LAMMPS to other codes
</H4>
<P>LAMMPS is designed to allow it to be coupled to other codes. For
example, a quantum mechanics code might compute forces on a subset of
atoms and pass those forces to LAMMPS. Or a continuum finite element
(FE) simulation might use atom positions as boundary conditions on FE
nodal points, compute a FE solution, and return interpolated forces on
MD atoms.
</P>
<P>LAMMPS can be coupled to other codes in at least 3 ways. Each has
advantages and disadvantages, which you'll have to think about in the
context of your application.
</P>
<P>(1) Define a new <A HREF = "fix.html">fix</A> command that calls the other code. In
this scenario, LAMMPS is the driver code. During its timestepping,
the fix is invoked, and can make library calls to the other code,
which has been linked to LAMMPS as a library. This is the way the
<A HREF = "http://www.rpi.edu/~anderk5/lab">POEMS</A> package that performs constrained rigid-body motion on
groups of atoms is hooked to LAMMPS. See the
<A HREF = "fix_poems.html">fix_poems</A> command for more details. See <A HREF = "Section_modify.html">this
section</A> of the documentation for info on how to add
a new fix to LAMMPS.
</P>
<P>(2) Define a new LAMMPS command that calls the other code. This is
conceptually similar to method (1), but in this case LAMMPS and the
other code are on a more equal footing. Note that now the other code
is not called during the timestepping of a LAMMPS run, but between
runs. The LAMMPS input script can be used to alternate LAMMPS runs
with calls to the other code, invoked via the new command. The
<A HREF = "run.html">run</A> command facilitates this with its <I>every</I> option, which
makes it easy to run a few steps, invoke the command, run a few steps,
invoke the command, etc.
</P>
<P>In this scenario, the other code can be called as a library, as in
(1), or it could be a stand-alone code, invoked by a system() call
made by the command (assuming your parallel machine allows one or more
processors to start up another program). In the latter case the
stand-alone code could communicate with LAMMPS thru files that the
command writes and reads.
</P>
<P>See <A HREF = "Section_modify.html">this section</A> of the documentation for how to
add a new command to LAMMPS.
</P>
<P>(3) Use LAMMPS as a library called by another code. In this case the
other code is the driver and calls LAMMPS as needed. Or a wrapper
code could link and call both LAMMPS and another code as libraries.
Again, the <A HREF = "run.html">run</A> command has options that allow it to be
invoked with minimal overhead (no setup or clean-up) if you wish to do
multiple short runs, driven by another program.
</P>
<P><A HREF = "Section_start.html#2_4">This section</A> of the documentation describes
how to build LAMMPS as a library. Once this is done, you can
interface with LAMMPS either via C++, C, or Fortran (or any other
language that supports a vanilla C-like interface, e.g. a scripting
language). For example, from C++ you could create one (or more)
"instances" of LAMMPS, pass it an input script to process, or execute
individual commands, all by invoking the correct class methods in
LAMMPS. From C or Fortran you can make function calls to do the same
things. Library.cpp and library.h contain such a C interface with the
functions:
</P>
<PRE>void lammps_open(int, char **, MPI_Comm, void **);
void lammps_close(void *);
void lammps_file(void *, char *);
char *lammps_command(void *, char *);
</PRE>
<P>The functions contain C++ code you could write in a C++ application
that was invoking LAMMPS directly. Note that LAMMPS classes are
defined within a LAMMPS namespace (LAMMPS_NS) if you use them
from another C++ application.
</P>
<P>Two of the routines in library.cpp are of particular note. The
lammps_open() function initiates LAMMPS and takes an MPI communicator
as an argument. It returns a pointer to a LAMMPS "object". As with
C++, the lammps_open() function can be called multiple times, to
create multiple instances of LAMMPS.
</P>
<P>LAMMPS will run on the set of processors in the communicator. This
means the calling code can run LAMMPS on all or a subset of
processors. For example, a wrapper script might decide to alternate
between LAMMPS and another code, allowing them both to run on all the
processors. Or it might allocate half the processors to LAMMPS and
half to the other code and run both codes simultaneously before
syncing them up periodically.
</P>
<P>Library.cpp contains a lammps_command() function to which the caller
passes a single LAMMPS command (a string). Thus the calling code can
read or generate a series of LAMMPS commands (e.g. an input script)
one line at a time and pass it thru the library interface to setup a
problem and then run it.
</P>
<P>A few other sample functions are included in library.cpp, but the key
idea is that you can write any functions you wish to define an
interface for how your code talks to LAMMPS and add them to
library.cpp and library.h. The routines you add can access any LAMMPS
data. The examples/couple directory has example C++ and C codes which
show how a stand-alone code can link LAMMPS as a library, run LAMMPS
on a subset of processors, grab data from LAMMPS, change it, and put
it back into LAMMPS.
</P>
<HR>
<A NAME = "4_11"></A><H4>4.11 Visualizing LAMMPS snapshots
</H4>
<P>LAMMPS itself does not do visualization, but snapshots from LAMMPS
simulations can be visualized (and analyzed) in a variety of ways.
</P>
<P>LAMMPS snapshots are created by the <A HREF = "dump.html">dump</A> command which can
create files in several formats. The native LAMMPS dump format is a
text file (see "dump atom" or "dump custom") which can be visualized
by the <A HREF = "Section_tools.html#xmovie">xmovie</A> program, included with the
LAMMPS package. This produces simple, fast 2d projections of 3d
systems, and can be useful for rapid debugging of simulation geometry
and atom trajectories.
</P>
<P>Several programs included with LAMMPS as auxiliary tools can convert
native LAMMPS dump files to other formats. See the
<A HREF = "Section_tools.html">Section_tools</A> doc page for details. The first is
the <A HREF = "Section_tools.html#charmm">ch2lmp tool</A>, which contains a
lammps2pdb Perl script which converts LAMMPS dump files into PDB
files. The second is the <A HREF = "Section_tools.html#arc">lmp2arc tool</A> which
converts LAMMPS dump files into Accelrys' Insight MD program files.
The third is the <A HREF = "Section_tools.html#cfg">lmp2cfg tool</A> which converts
LAMMPS dump files into CFG files which can be read into the
<A HREF = "http://164.107.79.177/Archive/Graphics/A">AtomEye</A> visualizer.
</P>
<P>A Python-based toolkit distributed by our group can read native LAMMPS
dump files, including custom dump files with additional columns of
user-specified atom information, and convert them to various formats
or pipe them into visualization software directly. See the <A HREF = "http://www.cs.sandia.gov/~sjplimp/pizza.html">Pizza.py
WWW site</A> for details. Specifically, Pizza.py can convert
LAMMPS dump files into PDB, XYZ, <A HREF = "http://www.ensight.com">Ensight</A>, and VTK formats.
Pizza.py can pipe LAMMPS dump files directly into the Raster3d and
RasMol visualization programs. Pizza.py has tools that do interactive
3d OpenGL visualization and one that creates SVG images of dump file
snapshots.
</P>
<P>LAMMPS can create XYZ files directly (via "dump xyz") which is a
simple text-based file format used by many visualization programs
including <A HREF = "http://www.ks.uiuc.edu/Research/vmd">VMD</A>.
</P>
<P>LAMMPS can create DCD files directly (via "dump dcd") which can be
read by <A HREF = "http://www.ks.uiuc.edu/Research/vmd">VMD</A> in conjunction with a CHARMM PSF file. Using this
form of output avoids the need to convert LAMMPS snapshots to PDB
files. See the <A HREF = "dump.html">dump</A> command for more information on DCD
files.
</P>
<P>LAMMPS can create XTC files directly (via "dump xtc") which is GROMACS
file format which can also be read by <A HREF = "http://www.ks.uiuc.edu/Research/vmd">VMD</A> for visualization.
See the <A HREF = "dump.html">dump</A> command for more information on XTC files.
</P>
<HR>
<A NAME = "4_12"></A><H4>4.12 Non-orthogonal simulation boxes
</H4>
<P>By default, LAMMPS uses an orthogonal simulation box to encompass the
particles. The <A HREF = "boundary.html">boundary</A> command sets the boundary
conditions of the box (periodic, non-periodic, etc). If the box size
is xprd by yprd by zprd then the 3 mutually orthogonal edge vectors of
an orthogonal simulation box are a = (xprd,0,0), b = (0,yprd,0), and c
= (0,0,zprd).
</P>
<P>LAMMPS also allows non-orthogonal simulation boxes (triclinic
symmetry) to be defined with 3 additional "tilt" parameters which
change the edge vectors of the simulation box to be a = (xprd,0,0), b
= (xy,yprd,0), and c = (xz,yz,zprd). The xy, xz, and yz parameters
can be positive or negative. The simulation box must be periodic in
both dimensions associated with a tilt factor. For example, if xz !=
0.0, then the x and z dimensions must be periodic.
</P>
<P>To avoid extremely tilted boxes (which would be computationally
inefficient), no tilt factor can skew the box more than half the
distance of the parallel box length, which is the 1st dimension in the
tilt factor (x for xz). For example, if xlo = 2 and xhi = 12, then
the x box length is 10 and the xy tilt factor must be between -5 and
5. Similarly, both xz and yz must be between -(xhi-xlo)/2 and
+(yhi-ylo)/2. Note that this is not a limitation, since if the
maximum tilt factor is 5 (as in this example), then configurations
with tilt = ..., -15, -5, 5, 15, 25, ... are all equivalent.
</P>
<P>You tell LAMMPS to use a non-orthogonal box when the simulation box is
defined. This happens in one of 3 ways. If the
<A HREF = "create_box.html">create_box</A> command is used with a region of style
<I>prism</I>, then a non-orthogonal domain is setup. See the
<A HREF = "region.html">region</A> command for details. If the
<A HREF = "read_data.html">read_data</A> command is used to define the simulation
box, and the header of the data file contains a line with the "xy xz
yz" keyword, then a non-orthogonal domain is setup. See the
<A HREF = "read_data.html">read_data</A> command for details. Finally, if the
<A HREF = "read_restart.html">read_restart</A> command reads a restart file which
was written from a simulation using a triclinic box, then a
non-orthogonal box will be enabled for the restarted simulation.
</P>
<P>Note that you can define a non-orthogonal box with all 3 tilt factors
= 0.0, so that it is initially orthogonal. This is necessary if the
box will become non-orthogonal. Alternatively, you can use the
<A HREF = "change_box.html">change_box</A> command to convert a simulation box from
orthogonal to non-orthogonal and vice versa.
</P>
<P>One use of non-orthogonal boxes is to model solid-state crystals with
triclinic symmetry. The <A HREF = "lattice.html">lattice</A> command can be used
with non-orthogonal basis vectors to define a lattice that will tile a
non-orthogonal simulation box via the <A HREF = "create_atoms.html">create_atoms</A>
command. Note that while the box edge vectors a,b,c cannot be
arbitrary vectors (e.g. a must be aligned with the x axis), it is
possible to rotate any crystal's basis vectors so that they meet these
restrictions.
</P>
<P>A second use of non-orthogonal boxes is to shear a bulk solid to study
the response of the material. The <A HREF = "fix_deform.html">fix deform</A>
command can be used for this purpose. It allows dynamic control of
the xy, xz, and yz tilt factors as a simulation runs.
</P>
<P>Another use of non-orthogonal boxes is to perform non-equilibrium MD
(NEMD) simulations, as discussed in the next section.
</P>
<HR>
<A NAME = "4_13"></A><H4>4.13 NEMD simulations
</H4>
<P>Non-equilibrium molecular dynamics or NEMD simulations are typically
used to measure a fluid's rheological properties such as viscosity.
In LAMMPS, such simulations can be performed by first setting up a
non-orthogonal simulation box (see the preceding Howto section).
</P>
<P>A shear strain can be applied to the simulation box at a desired
strain rate by using the <A HREF = "fix_deform.html">fix deform</A> command. The
<A HREF = "fix_nvt_sllod.html">fix nvt/sllod</A> command can be used to thermostat
the sheared fluid and integrate the SLLOD equations of motion for the
system. Fix nvt/sllod uses <A HREF = "compute_temp_deform.html">compute
temp/deform</A> to compute a thermal temperature
by subtracting out the streaming velocity of the shearing atoms. The
velocity profile or other properties of the fluid can be monitored via
the <A HREF = "fix_ave_spatial.html">fix ave/spatial</A> command.
</P>
<P>As discussed in the previous section on non-orthogonal simulation
boxes, the amount of tilt or skew that can be applied is limited by
LAMMPS for computational efficiency to be 1/2 of the parallel box
length. However, <A HREF = "fix_deform.html">fix deform</A> can continuously strain
a box by an arbitrary amount. As discussed in the <A HREF = "fix_deform.html">fix
deform</A> command, when the tilt value reaches a limit,
the box is re-shaped to the opposite limit which is an equivalent
tiling of periodic space. The strain rate can then continue to change
as before. In a long NEMD simulation these box re-shaping events may
occur many times.
</P>
<P>In a NEMD simulation, the "remap" option of <A HREF = "fix_deform.html">fix
deform</A> should be set to "remap v", since that is what
<A HREF = "fix_nvt_sllod.html">fix nvt/sllod</A> assumes to generate a velocity
profile consistent with the applied shear strain rate.
</P>
<P>An alternative method for calculating viscosities is provided via the
<A HREF = "fix_viscosity.html">fix viscosity</A> command.
</P>
<HR>
<A NAME = "4_14"></A><H4>4.14 Extended spherical and aspherical particles
</H4>
<P>Typical MD models treat atoms or particles as point masses.
Sometimes, however, it is desirable to have a model where the
particles are extended spherioids or extended aspherical paticles such
as an ellipsoid. The difference is that such particles have a moment
of inertia, rotational energy, and angular momentum. Rotation is
induced by torque from interactions with other particles.
</P>
<P>LAMMPS has several options for running simulations with these kinds of
particles. The following aspects are discussed in turn:
</P>
<UL><LI>atom styles
<LI>pair potentials
<LI>time integration
<LI>computes, thermodynamics, and dump output
<LI>rigid bodies composed of extended particles
</UL>
<H5>Atom styles
</H5>
<P>There are 3 <A HREF = "atom_style.html">atom styles</A> that define extended
particles: granular, dipole, ellipsoid.
</P>
<P>Granular particles are extended spheriods and each particle can have a
unique diameter and mass (or density). These particles store
an angular velocity (omega) and can be acted upon by torque.
</P>
<P>Dipole particles are extended spheriods with a point dipole and each
particle type has a diamater and mass, set by the <A HREF = "shape.html">shape</A>
and <A HREF = "mass.html">mass</A> commands. These particles store an angular
velocity (omega) and can be acted upon by torque. They also store an
orientation for the point dipole (mu) which has a length set by the
<A HREF = "dipole.html">dipole</A> command. The <A HREF = "set.html">set</A> command can be used
to initialize the orientation of dipole moments.
</P>
<P>Ellipsoid particles are aspherical. Each particle type has an
ellipsoidal shape and mass, defined by the <A HREF = "shape.html">shape</A> and
<A HREF = "mass.html">mass</A> commands. These particles store an angular momentum
and their orientation (quaternion), and can be acted upon by torque.
They do not store an angular velocity (omega) which can be in a
different direction than angular momentum. The <A HREF = "set.html">set</A> command
can be used to initialize the orientation of ellipsoidal particles and
has a brief explanation of quaternions.
</P>
<P>Note that if one of these atom styles is used (or multiple styles via
the <A HREF = "atom_style.html">atom_style hybrid</A> command), not all particles in
the system are required to be extended or aspherical. For example, if
the 3 shape parameters are set to the same value, the particle will be
a spheroid rather than an ellipsoid. If the dipole moment is set to
zero, the particle will not have a point dipole associated with it.
The pair styles used to compute pairwise interactions will typically
compute the correct interaction in these simplified (cheaper) cases.
<A HREF = "pair_hybrid.html">Pair_style hybrid</A> can be used to insure the correct
interactions are computed for the appropriate style of interactions.
Likewise, using groups to partition particles (ellipsoid versus
spheroid versus point particles) will allow you to use the appropriate
time integrators and temperature computations for each class of
particles. See the doc pages for various commands for details.
</P>
<P>Also note that for <A HREF = "dimension.html">2d simulations</A>, extended spheroids
and ellipsoids are still treated as 3d particles, rather than as disks
or ellipses. This means they still have a moment of inertia for a 3d
extended object. When their temperature is coomputed, the correct
degrees of freedom are used for rotation in a 2d versus 3d system.
</P>
<H5>Pair potentials
</H5>
<P>When a system with extended particles is defined, the particles will
only rotate and experience torque if the force field computes such
interactions. These are the various <A HREF = "pair_style.html">pair styles</A>
that generate torque:
</P>
<UL><LI><A HREF = "pair_gran.html">pair_style gran/history</A>
<LI><A HREF = "pair_gran.html">pair_style gran/hertzian</A>
<LI><A HREF = "pair_gran.html">pair_style gran/no_history</A>
<LI><A HREF = "pair_dipole.html">pair_style dipole/cut</A>
<LI><A HREF = "pair_gayberne.html">pair_style gayberne</A>
<LI><A HREF = "pair_resuared.html">pair_style resquared</A>
<LI><A HREF = "pair_lubricate.html">pair_style lubricate</A>
</UL>
<P>The <A HREF = "pair_gran.html">granular pair styles</A> are used with <A HREF = "atom_style.html">atom_style
granular</A>. The <A HREF = "pair_dipole.html">dipole pair style</A>
is used with <A HREF = "atom_style.html">atom_style dipole</A>. The
<A HREF = "pair_gayberne.html">GayBerne</A> and <A HREF = "pair_resquared.html">REsquared</A>
potentials require particles have a <A HREF = "shape.html">shape</A> and are
designed for <A HREF = "atom_style.html">ellipsoidal particles</A>. The
<A HREF = "pair_lubricate.html">lubrication potential</A> requires that particles
have a <A HREF = "shape.html">shape</A>. It can currently only be used with
extended spherical particles.
</P>
<H5>Time integration
</H5>
<P>There are 3 fixes that perform time integration on extended spherical
particles, meaning the integrators update the rotational orientation
and angular velocity or angular momentum of the particles:
</P>
<UL><LI><A HREF = "fix_nve_sphere.html">fix nve/sphere</A>
<LI><A HREF = "fix_nvt_sphere.html">fix nvt/sphere</A>
<LI><A HREF = "fix_npt_sphere.html">fix npt/sphere</A>
</UL>
<P>Likewise, there are 3 fixes that perform time integration on extended
aspherical particles:
</P>
<UL><LI><A HREF = "fix_nve_asphere.html">fix nve/asphere</A>
<LI><A HREF = "fix_nvt_asphere.html">fix nvt/asphere</A>
<LI><A HREF = "fix_npt_asphere.html">fix npt/asphere</A>
</UL>
<P>The advantage of these fixes is that those which thermostat the
particles include the rotational degrees of freedom in the temperature
calculation and thermostatting. Other thermostats can be used with
fix nve/sphere or fix nve/asphere, such as fix langevin or fix
temp/berendsen, but those thermostats only operate on the
translational kinetic energy of the extended particles.
</P>
<P>Note that for mixtures of point and extended particles, you should
only use these integration fixes on <A HREF = "group.html">groups</A> which contain
extended particles.
</P>
<H5>Computes, thermodynamics, and dump output
</H5>
<P>There are 4 computes that calculate the temperature or rotational energy
of extended spherical or aspherical particles:
</P>
<UL><LI><A HREF = "compute_temp_sphere.html">compute temp/sphere</A>
<LI><A HREF = "compute_temp_asphere.html">compute temp/asphere</A>
<LI><A HREF = "compute_erotate_sphere.html">compute erotate/sphere</A>
<LI><A HREF = "compute_erotate_asphere.html">compute erotate/asphere</A>
</UL>
<P>These include rotational degrees of freedom in their computation. If
you wish the thermodynamic output of temperature or pressure to use
one of these computes (e.g. for a system entirely composed of extended
particles), then the compute can be defined and the
<A HREF = "thermo_modify.html">thermo_modify</A> command used. Note that by
default thermodynamic quantities will be calculated with a temperature
that only includes translational degrees of freedom. See the
<A HREF = "thermo_style.html">thermo_style</A> command for details.
</P>
<P>The <A HREF = "dump.html">dump custom</A> command can output various attributes of
extended particles, including the dipole moment (mu), the angular
velocity (omega), the angular momentum (angmom), the quaternion
(quat), and the torque (tq) on the particle.
</P>
<H5>Rigid bodies composed of extended particles
</H5>
<P>The <A HREF = "fix_rigid.html">fix rigid</A> command treats a collection of
particles as a rigid body, computes its inertia tensor, sums the total
force and torque on the rigid body each timestep due to forces on its
constituent particles, and integrates the motion of the rigid body.
</P>
<P>(NOTE: 6/08 the feature described in the following paragraph has
not yet been released. It will be soon.)
</P>
<P>If any of the constituent particles of a rigid body are extended
particles (spheroids or ellipsoids), then their contribution to the
inertia tensor of the body is different than if they were point
particles. This means the rotational dynamics of the rigid body will
be different. Thus a model of a dimer is different if the dimer
consists of two point masses versus two extended sphereoids, even if
the two particles have the same mass. Extended particles that
experience torque due to their interaction with other particles will
also impart that torque to a rigid body they are part of.
</P>
<P>See the "fix rigid" command for example of complex rigid-body models
it is possible to define in LAMMPS.
</P>
<P>Note that the <A HREF = "fix_shake.html">fix shake</A> command can also be used to
treat 2, 3, or 4 particles as a rigid body, but it always assumes the
particles are point masses.
</P>
<HR>
<A NAME = "4_15"></A><H4>4.15 Output from LAMMPS
</H4>
<P>Aside from <A HREF = "restart.html">restart files</A>, there are two basic kinds of
LAMMPS output. The first is <A HREF = "thermo_style.html">thermodynamic output</A>,
which is a list of quantities printed every few timesteps to the
screen and logfile. The second is <A HREF = "dump.html">dump files</A>, which
contain snapshots of atoms and various per-atom values and are written
at a specified frequency. A simulation prints one set of
thermodynamic output; it may generate any number of dump files. As
discussed below, LAMMPS gives you a variety of ways to determine what
quantities are computed and printed when thermodynamic info or dump
files are output. There are also three fixes which can do their own
output of user-chosen quantities: <A HREF = "fix_ave_time.html">fix ave/time</A> for
time averaging, <A HREF = "fix_ave_spatial.html">fix ave/spatial</A> for spatial
averaging, and <A HREF = "fix_print.html">fix print</A>. These are also described
below. Throughout this discussion, note that users can <A HREF = "Section_modify.html">add their own
computes and fixes to LAMMPS</A> which can then
generate values that can be output with these commands.
</P>
<H5>Thermodynamic output
</H5>
<P>The frequency and format of thermodynamic output is set by the
<A HREF = "thermo.html">thermo</A>, <A HREF = "thermo_style.html">thermo_style</A>, and
<A HREF = "thermo_modify.html">thermo_modify</A> commands. The
<A HREF = "thermo_style.html">thermo_style</A> command also specifies what values are
calculated and written out. Pre-defined keywords can be specified
(e.g. press, etotal, etc). Three additional kinds of keywords can also
be specified (c_ID, f_ID, v_name), where a <A HREF = "compute.html">compute</A> or
<A HREF = "fix.html">fix</A> or <A HREF = "variable.html">variable</A> provides the value to be
output. Each of these are described in turn.
</P>
<P>In LAMMPS, a <A HREF = "compute.html">compute</A> comes in two flavors: ones that
compute global values (e.g. temperature, pressure tensor) and ones
that compute per-atom values. Only global quantities from a compute
can be used for thermodynamic output. The user-defined ID of the
compute is used along with an optional subscript as part of the
<A HREF = "thermo_style.html">thermo_style</A> command. E.g. c_myTemp outputs the
single scalar value generated by the compute; c_myTemp[2] outputs
the 2nd vector value. Note that there is a <A HREF = "compute_reduce.html">compute
reduce</A> command which can sum per-atom quantities
into a global scalar or vector.
</P>
<P><A HREF = "fix.html">Fixes</A> can generate global scalar or vector values which can
be output with thermodynamic output, e.g. the energy of an indenter's
interaction with the simulation atoms. These values are accessed via
the same format as a compute's values, as f_ID or f_ID[N]. See the
doc pages for individual fix commands to see which ones generate
global values that can be output with thermodynamic info. The <A HREF = "fix_ave_time.html">fix
ave/time</A> command generates time-averaged global
quantities which can be accessed for thermodynamic output, e.g. a
time-averaged pressure.
</P>
<P><A HREF = "variable.html">Variables</A> can be defined in the input script. All
styles except the atom-style variable can be used for thermodynamic
output, since it generates per-atom values. A variable with the name
"abc" is referenced in a thermo_style command as v_abc.
</P>
<P>The formula associated with equal-style variables can contain math
operations and functions (x+y,x*y,(),sqrt,exp,etc), atom values
(x[N],fx[N],etc), group functions (mass(),vcm(),etc), references
to thermodynamic quantities (temp,press,vol,etc), references to
<A HREF = "compute.html">computes</A> that generate global values, references to
<A HREF = "fix.html">fixes</A> that generate global values, or references to other
<A HREF = "variable.html">variables</A>. Thus an equal-style variable is the most
general way to define some quantity to calculate and include with
thermodynamic output.
</P>
<H5>Dump file output
</H5>
<P>Dump file output is specified by the <A HREF = "dump.html">dump</A> and
<A HREF = "dump_modify.html">dump_modify</A> commands. There are several
pre-defined formats (dump atom, dump xtc, etc). There is also a <A HREF = "dump.html">dump
custom</A> format where the user specifies what values are
output with each atom. Pre-defined keywords can be specified (tag, x,
fx, etc). Three additional kinds of keywords can also be specified
(c_ID, f_ID, v_name), where a <A HREF = "compute.html">compute</A> or <A HREF = "fix.html">fix</A>
or <A HREF = "variable.html">variable</A> provides the value to be output. Each of
these are described in turn.
</P>
<P><A HREF = "compute.html">Computes</A> that generate one or more per-atom values can
be accessed by the dump custom command. These are computes that have
the word "atom" in their style name, e.g. ke/atom, stress/atom, etc.
The values are accessed as c_myKE for a scalar per-atom quantity or as
c_myStress[2] for a component of a vector per-atom quantity.
</P>
<P><A HREF = "fix.html">Fixes</A> can generate per-atom values to output to dump files.
For example, the <A HREF = "fix_ave_atom.html">fix ave/atom</A> command calculates
time-averages of per-atom quantities, such as per-atom
<A HREF = "compute.html">computes</A> and atom-style <A HREF = "variable.html">variables</A>.
These per-atom fix values are accessed by the <A HREF = "dump.html">dump custom</A>
command via the same format as a compute's values, as f_myKE or
f_myStress[2].
</P>
<P><A HREF = "variable.html">Variables</A> can be defined in the input script. Only
atom-style variables can be used for dump custom output, since only
they produce per-atom values. A variable with the name "abc" is
referenced in a dump custom command as v_abc.
</P>
<P>Just like equal-style variables, the formula associated with
atom-style variables can contain math operations and functions
(x+y,x*y,(),sqrt,exp,etc), atom values (x[N],fx[N],etc), group
functions (mass(),vcm(),etc), references to thermodynamic quantities
(temp,press,vol,etc), references to <A HREF = "compute.html">computes</A> that
generate global values, references to <A HREF = "fix.html">fixes</A> that generate
global values, or references to non atom-style variables that generate
global values. In addition, an atom-style variable can reference
vectors of atom values (x[],fx[],etc), <A HREF = "compute.html">computes</A>
that generate per-atom values, <A HREF = "fix.html">fixes</A> that generate per-atom
values, or other atom-style variables. Thus an atom-style variable is
the most general way to define some quantity to calculate and output
to a dump file.
</P>
<H5>Fix output
</H5>
<P>Three other fixes are of particular note for output: <A HREF = "fix_ave_time.html">fix
ave/time</A>, <A HREF = "fix_ave_spatial.html">fix ave/spatial</A>,
and <A HREF = "fix_print.html">fix print</A>.
</P>
<P>The <A HREF = "fix_ave_time.html">fix ave/time</A> command enables time-averaging of
any global quantity, like those output with thermodynamic info. The
user specifies one or more quantities to time average. These can be
global <A HREF = "compute.html">compute</A> values, global <A HREF = "fix.html">fix</A> values, or
<A HREF = "variable.html">variables</A> of any style except the atom style which
produces per-atom values. Since a variable can refer to keywords used
by the <A HREF = "thermo_style.html">thermo_style custom</A> command (like temp or
press), any thermodynamic quantity can be time averaged in this way.
</P>
<P>The <A HREF = "fix_ave_time.html">fix ave/time</A> command offers several options
for how it performs time averaging. The results it produces can be
used in two ways. First, they can be written directly to a file, one
line per timestamp. Note that the averaging parameters can be
specified in such a way that averaging is not done at all, in which
case this is simply a convenient means of outputting desired
quantities directly to a separate file. Like other fixes that produce
global quantities, the results of this fix can also be used as input
to any command that accesses global quantities, e.g. by the
<A HREF = "thermo_style.html">thermo_style custom</A> command, by a variable, etc.
</P>
<P>The <A HREF = "fix_ave_spatial.html">fix ave/spatial</A> command enables
spatial-averaging of per-atom quantities like those output in dump
files, within 1d layers of the simulation box. The per-atom
quantities can be atom density (mass or number) or atom attributes
such as position, velocity, force. They can also be per-atom
quantities calculated by a <A HREF = "compute.html">compute</A>, by a
<A HREF = "fix.html">fix</A>, or by an atom-style <A HREF = "variable.html">variable</A>.
</P>
<P>The <A HREF = "fix_ave_spatial.html">fix ave/spatial</A> command offers several
options for how it performs time averaging. The per-layer values it
produces can be used in two ways. First, they can be written directly
to a file. Note that the averaging parameters can be specified in
such a way that time averaging is not done, in which case this is a
convenient means of simply outputting desired quantities (summed over
atoms within a 1d layer) directly to a separate file. Like other
fixes that produce global quantities, the results of this fix can also
be used as input by any command that accesses global quantities,
e.g. by the <A HREF = "thermo_style.html">thermo_style custom</A> command, by a
variable, etc. See the doc page for <A HREF = "fix_ave_spatial.html">fix
ave/spatial</A> for a description of how these
values are indexed.
</P>
<P>The <A HREF = "fix_print.html">fix print</A> command can generate a line of output
written to the screen and log file or to a separate file, periodically
during a running simulation. The line can contain one or more
<A HREF = "variable.html">variable</A> values (for any style variable except the
atom style), and as explained above, variables themselves can contain
references to global values generated by <A HREF = "thermo_style.html">thermodynamic
keywords</A>, <A HREF = "compute.html">computes</A>,
<A HREF = "fix.html">fixes</A>, or other <A HREF = "variable.html">variables</A>. Thus the <A HREF = "fix_print.html">fix
print</A> command is a means to output any desired
calculated quantity separate from normal thermodynamic or dump file
output.
</P>
<P>This table summarizes the various output options, specifying what
their inputs and outputs are. The frequency with which they are
invoked and produce output is also listed. Basically, any two
commands can be hooked together so long as one produces output that
matches the input needs of the other. A "match" means that the
frequencies and global vs per-atom attributes are the same.
</P>
<DIV ALIGN=center><TABLE BORDER=1 >
<TR><TD >Command</TD><TD > Input</TD><TD > Input Freq</TD><TD > Output</TD><TD > Output Freq</TD></TR>
<TR><TD >thermo_style custom</TD><TD > thermo keyword, global scalar/vector compute, global scalar/vector fix, equal variable</TD><TD > nthermo</TD><TD > screen, log</TD><TD > nthermo</TD></TR>
<TR><TD >dump custom</TD><TD > keyword, peratom scalar/vector compute, peratom scalar/vector fix, atom variable</TD><TD > dump freq</TD><TD > file</TD><TD > dump freq</TD></TR>
<TR><TD >global fixes</TD><TD > N/A</TD><TD > N/A</TD><TD > global scalar/vector</TD><TD > 1 or nevery</TD></TR>
<TR><TD >peratom fixes</TD><TD > N/A</TD><TD > N/A</TD><TD > peratom scalar/vector</TD><TD > 1 or nevery</TD></TR>
<TR><TD >fix ave/time</TD><TD > global scalar/vector fix, global scalar/vector compute, equal variable</TD><TD > nevery</TD><TD > global scalar/vector, file</TD><TD > nfreq</TD></TR>
<TR><TD >fix ave/spatial</TD><TD > peratom scalar/vector fix, peratom scalar/vector compute, atom vector, atom variable, density mass/number</TD><TD > nevery</TD><TD > global vector</TD><TD > nfreq, file</TD></TR>
<TR><TD >fix ave/atom</TD><TD > peratom scalar/vector compute, peratom scalar/vector fix, atom variable, atom vector</TD><TD > nevery</TD><TD > peratom scalar/vector</TD><TD > nfreq</TD></TR>
<TR><TD >fix print</TD><TD > any variable</TD><TD > nevery</TD><TD > screen, file</TD><TD > nevery</TD></TR>
<TR><TD >global computes</TD><TD > N/A</TD><TD > N/A</TD><TD > global scalar/vector</TD><TD > N/A</TD></TR>
<TR><TD >peratom computes</TD><TD > N/A</TD><TD > N/A</TD><TD > peratom scalar/vector</TD><TD > N/A</TD></TR>
<TR><TD >compute sum</TD><TD > peratom scalar/vector compute, peratom scalar/vector fix, atom variable</TD><TD > N/A</TD><TD > global scalar/vector</TD><TD > N/A</TD></TR>
<TR><TD >variable equal</TD><TD > thermo keywords, atom value vx[123], global scalar/vector compute, global scalar/vector fix, non-atom variable</TD><TD > N/A</TD><TD > global scalar</TD><TD > N/A</TD></TR>
<TR><TD >variable atom</TD><TD > thermo keywords, atom value vx[123], atom vector vx[], global scalar/vector compute, peratom scalar/vector compute, global scalar/vector fix, peratom scalar/vector fix, any variable</TD><TD > N/A</TD><TD > peratom scalar</TD><TD > N/A</TD></TR>
<TR><TD >print</TD><TD > any variable</TD><TD > N/A</TD><TD > screen, log</TD><TD > between runs</TD></TR>
<TR><TD >run every</TD><TD > any variable</TD><TD > nevery</TD><TD > screen, log</TD><TD > nevery
</TD></TR></TABLE></DIV>
<HR>
<A NAME = "4_16"></A><H4>4.16 Thermostatting, barostatting, and computing temperature
</H4>
<P>Thermostatting means controlling the temperature of particles in an MD
simulation. Barostatting means controlling the pressure. Since the
pressure includes a kinetic component due to particle velocities, both
these operations require calculation of the temperature. Typically a
target temperature (T) and/or pressure (P) is specified by the user,
and the thermostat or barostat attempts to equilibrate the system to
the requested T and/or P.
</P>
<P>Temperature is computed as kinetic energy divided by some number of
degrees of freedom (and the Boltzmann constant). Since kinetic energy
is a function of particle velocity, there is often a need to
distinguish between a particle's advection velocity (due to some
aggregate motiion of particles) and its thermal velocity. The sum of
the two is the particle's total velocity, but the latter is often what
is wanted to compute a temperature.
</P>
<P>LAMMPS has several options for computing temperatures, any of which
can be used in thermostatting and barostatting. These <A HREF = "compute.html">compute
commands</A> calculate temperature, and the <A HREF = "compute_pressure.html">compute
pressure</A> command calculates pressure.
</P>
<UL><LI><A HREF = "compute_temp.html">compute temp</A>
<LI><A HREF = "compute_temp_sphere.html">compute temp/sphere</A>
<LI><A HREF = "compute_temp_asphere.html">compute temp/asphere</A>
<LI><A HREF = "compute_temp_com.html">compute temp/com</A>
<LI><A HREF = "compute_temp_deform.html">compute temp/deform</A>
<LI><A HREF = "compute_temp_partial.html">compute temp/partial</A>
<LI><A HREF = "compute_temp_ramp.html">compute temp/ramp</A>
<LI><A HREF = "compute_temp_region.html">compute temp/region</A>
</UL>
<P>All but the first 3 calculate velocity biases (i.e. advection
velocities) that are removed when computing the thermal temperature.
<A HREF = "fix_temp_sphere.html">Fix temp/sphere</A> and <A HREF = "fix_temp_asphere.html">fix
temp/asphere</A> compute kinetic energy for
extended particles that includes rotational degrees of freedom. They
both allow, as an extra argument, another temperature compute that
subtracts a velocity bias, so the translational velocity of extended
spherical or aspherical particles can be adjusted in prescribed ways.
</P>
<P>Thermostatting in LAMMPS is performed by <A HREF = "fix.html">fixes</A>. Four
thermostatting fixes are currently available: Nose-Hoover (nvt),
Berendsen, Langevin, and direct rescaling (temp/rescale):
</P>
<UL><LI><A HREF = "fix_nvt.html">fix nvt</A>
<LI><A HREF = "fix_nvt_sphere.html">fix nvt/sphere</A>
<LI><A HREF = "fix_nvt_asphere.html">fix nvt/asphere</A>
<LI><A HREF = "fix_nvt_sllod.html">fix nvt/sllod</A>
<LI><A HREF = "fix_temp_berendsen.html">fix temp/berendsen</A>
<LI><A HREF = "fix_langevin.html">fix langevin</A>
<LI><A HREF = "fix_temp_rescale.html">fix temp/rescale</A>
</UL>
<P><A HREF = "fix_nvt.html">Fix nvt</A> only thermostats the translational velocity of
particles. <A HREF = "fix_nvt_sllod.html">Fix nvt/sllod</A> does as well, except
that it subtracts out a velocity bias due to a deforming box and
integrates the SLLOD equations of motion. See the <A HREF = "#4_13">NEMD
simulations</A> section of this page for further details. <A HREF = "fix_nvt_sphere.html">Fix
nvt/sphere</A> and <A HREF = "fix_nvt_asphere.html">fix
nvt/asphere</A> thermostat not only translation
velocities but also rotational velocities for spherical and aspherical
particles.
</P>
<P>Any of these fixes can use temperature computes that remove bias for
two purposes: (a) computing the current temperature to compare to the
requested target temperature, and (b) adjusting only the thermal
temperature component of the particle's velocities. See the doc pages
for the individual fixes and for the <A HREF = "fix_modify.html">fix_modify</A>
command for instructions on how to assign a temperature compute to a
thermostatting fix. For example, you can apply a thermostat to only
the x and z components of velocity by using it in conjunction with
<A HREF = "compute_temp_partial.html">compute temp/partial</A>.
</P>
<P>IMPORTANT NOTE: Only the nvt fixes perform time integration, meaning
they update the velocities and positions of particles due to forces
and velocities respectively. The other thermostat fixes only adjust
velocities; they do NOT perform time integration. Thus they should be
used in conjunction with a constant NVE integration fix such as these:
</P>
<UL><LI><A HREF = "fix_nve.html">fix nve</A>
<LI><A HREF = "fix_nve_sphere.html">fix nve/sphere</A>
<LI><A HREF = "fix_nve_asphere.html">fix nve/asphere</A>
</UL>
<P>Barostatting in LAMMPS is also performed by <A HREF = "fix.html">fixes</A>. Two
barosttating methods are currently available: Nose-Hoover (npt and
nph) and Berendsen:
</P>
<UL><LI><A HREF = "fix_npt.html">fix npt</A>
<LI><A HREF = "fix_npt_sphere.html">fix npt/sphere</A>
<LI><A HREF = "fix_npt_asphere.html">fix npt/asphere</A>
<LI><A HREF = "fix_nph.html">fix nph</A>
<LI><A HREF = "fix_press_berendsen.html">fix press/berendsen</A>
</UL>
<P>The <A HREF = "fix_npt.html">fix npt</A> commands include a Nose-Hoover thermostat
and barostat. <A HREF = "fix_nph.html">Fix nph</A> is just a Nose/Hoover barostat;
it does no thermostatting. Both <A HREF = "fix_nph.html">fix nph</A> and <A HREF = "fix_press_berendsen.html">fix
press/bernendsen</A> can be used in conjunction
with any of the thermostatting fixes.
</P>
<P>As with the thermostats, <A HREF = "fix_npt.html">fix npt</A> and <A HREF = "fix_nph.html">fix
nph</A> only use translational motion of the particles in
computing T and P and performing thermo/barostatting. <A HREF = "fix_npt_sphere.html">Fix
npt/sphere</A> and <A HREF = "fix_npt_asphere.html">fix
npt/asphere</A> thermo/barostat using not only
translation velocities but also rotational velocities for spherical
and aspherical particles.
</P>
<P>All of the barostatting fixes use the <A HREF = "compute_pressure.html">compute
pressure</A> compute to calculate a current
pressure. The barostatting fixes can also use temperature computes
that remove bias for the purpose of computing the current temperature
which contributes to the current pressure. See the doc pages for the
individual fixes and for the <A HREF = "fix_modify.html">fix_modify</A> command for
instructions on how to assign a temperature or pressure compute to a
barostatting fix.
</P>
<P>IMPORTANT NOTE: As with the thermostats, the Nose/Hoover methods (<A HREF = "fix_npt.html">fix
npt</A> and <A HREF = "fix_nph.html">fix nph</A>) perform time
integration. <A HREF = "fix_press_berendsen.html">Fix press/berendsen</A> does NOT,
so it should be used with one of the constant NVE fixes or with one of
the NVT fixes.
</P>
<P>Finally, thermodynamic output, which can be setup via the
<A HREF = "thermo_style.html">thermo_style</A> command, often includes temperature
and pressure values. As explained on the doc page for the
<A HREF = "thermo_style.html">thermo_style</A> command, the default T and P are
setup by the thermo command itself. They are NOT the ones associated
with any thermostatting or barostatting fix you have defined or with
any compute that calculates a temperature or pressure. Thus if you
want to view these values of T and P, you need to specify them
explicitly via a <A HREF = "thermo_style.html">thermo_style custom</A> command. Or
you can use the <A HREF = "thermo_modify.html">thermo_modify</A> command to
re-define what temperature or pressure compute is used for default
thermodynamic output.
</P>
<HR>
<HR>
<A NAME = "Cornell"></A>
<P><B>(Cornell)</B> Cornell, Cieplak, Bayly, Gould, Merz, Ferguson,
Spellmeyer, Fox, Caldwell, Kollman, JACS 117, 5179-5197 (1995).
</P>
<A NAME = "Horn"></A>
<P><B>(Horn)</B> Horn, Swope, Pitera, Madura, Dick, Hura, and Head-Gordon,
J Chem Phys, 120, 9665 (2004).
</P>
<A NAME = "MacKerell"></A>
<P><B>(MacKerell)</B> MacKerell, Bashford, Bellott, Dunbrack, Evanseck, Field,
Fischer, Gao, Guo, Ha, et al, J Phys Chem, 102, 3586 (1998).
</P>
<A NAME = "Jorgensen"></A>
<P><B>(Jorgensen)</B> Jorgensen, Chandrasekhar, Madura, Impey, Klein, J Chem
Phys, 79, 926 (1983).
</P>
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