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<CENTER><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>
</CENTER>
<HR>
<H3>fix command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID style args
</PRE>
<UL><LI>ID = user-assigned name for the fix
<LI>group-ID = ID of the group of atoms to apply the fix to
<LI>style = one of a long list of possible style names (see below)
<LI>args = arguments used by a particular style
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all nve
fix 3 all nvt 300.0 300.0 0.01
fix mine top setforce 0.0 NULL 0.0
</PRE>
<P><B>Description:</B>
</P>
<P>Set a fix that will be applied to a group of atoms. In LAMMPS, a
"fix" is any operation that is applied to the system during
timestepping or minimization. Examples include updating of atom
positions and velocities due to time integration, controlling
temperature, applying constraint forces to atoms, enforcing boundary
conditions, computing diagnostics, etc. There are dozens of fixes
defined in LAMMPS and new ones can be added - see <A HREF = "Section_modify.html">this
section</A> for a discussion.
</P>
<P>Each fix style has its own documentation page which describes its
arguments and what it does, as listed below.
</P>
<P>Fixes perform their operations at different stages of the timestep.
If 2 or more fixes both operate at the same stage of the timestep,
they are invoked in the order they were specified in the input script.
</P>
<P>Fixes can be deleted with the <A HREF = "unfix.html">unfix</A> command. Note that
this is the only way to turn off a fix; simply specifying a new fix
with a similar style will not turn off the first one. For example,
using a <A HREF = "fix_nve.html">fix nve</A> command for a second run after using a
<A HREF = "fix_nvt.html">fix nvt</A> command for the first run, will not cancel out
the NVT time integration invoked by the "fix nvt" command. Thus two
time integrators would be in place!
</P>
<P>If you specify a new fix with the same ID and style as an existing
fix, the old fix is deleted and the new one is created (presumably
with new settings). This is the same as if an "unfix" command were
first performed on the old fix, except that the new fix is kept in the
same order relative to the existing fixes as the old one originally
was. Note that this operation also wipes out any additional changes
made to the old fix via the <A HREF = "fix_modify.html">fix_modify</A> command.
</P>
<P>The <A HREF = "fix_modify.html">fix modify</A> command allows settings for some
fixes to be reset. See the doc page for individual fixes for details.
</P>
<P>Some fixes calculate a global scalar or vector quantity which can be
accessed by various output commands, including
<A HREF = "variable.html">variables</A>, <A HREF = "thermo_style.html">thermo_style custom</A>,
and <A HREF = "fix_ave_time.html">fix ave/time</A>. See this <A HREF = "Section_howto.html#4_15">howto
section</A> for a summary of various LAMMPS
output options. See the doc pages for individual fixes for info on
which ones calculate these quantities.
</P>
<P>Some fixes store an internal "state" which is written to binary
restart files via the <A HREF = "restart.html">restart</A> or
<A HREF = "write_restart.html">write_restart</A> commands. This allows the fix to
continue on with its calculations in a restarted simulation. See the
<A HREF = "read_restart.html">read_restart</A> command for info on how to re-specify
a fix in an input script that reads a restart file. See the doc pages
for individual fixes for info on which ones can be restarted.
</P>
<P>Here is an alphabetic list of fix styles available in LAMMPS:
</P>
<UL><LI><A HREF = "fix_addforce.html">addforce</A> - add a force to each atom
<LI><A HREF = "fix_aveforce.html">aveforce</A> - add an averaged force to each atom
<LI><A HREF = "fix_ave_atom.html">ave/atom</A> - compute per-atom time-averaged quantities
<LI><A HREF = "fix_ave_spatial.html">ave/spatial</A> - output per-atom quantities by layer
<LI><A HREF = "fix_ave_time.html">ave/time</A> - output time-averaged compute quantities
<LI><A HREF = "fix_com.html">com</A> - compute a center-of-mass
<LI><A HREF = "fix_deform.html">deform</A> - change the simulation box size/shape
<LI><A HREF = "fix_deposit.html">deposit</A> - add new atoms above a surface
<LI><A HREF = "fix_drag.html">drag</A> - drag atoms towards a defined coordinate
<LI><A HREF = "fix_dt_reset.html">dt/reset</A> - reset the timestep based on velocity, forces
<LI><A HREF = "fix_efield.html">efield</A> - impose electric field on system
<LI><A HREF = "fix_enforce2d.html">enforce2d</A> - zero out z-dimension velocity and force
<LI><A HREF = "fix_freeze.html">freeze</A> - freeze atoms in a granular simulation
<LI><A HREF = "fix_gravity.html">gravity</A> - add gravity to atoms in a granular simulation
<LI><A HREF = "fix_gyration.html">gyration</A> - compute radius of gyration
<LI><A HREF = "fix_heat.html">heat</A> - add/subtract momentum-conserving heat
<LI><A HREF = "fix_indent.html">indent</A> - impose force due to an indenter
<LI><A HREF = "fix_langevin.html">langevin</A> - Langevin temperature control
<LI><A HREF = "fix_lineforce.html">lineforce</A> - constrain atoms to move in a line
<LI><A HREF = "fix_msd.html">msd</A> - compute mean-squared displacement (i.e. diffusion coefficient)
<LI><A HREF = "fix_momentum.html">momentum</A> - zero the linear and/or angular momentum of a group of atoms
<LI><A HREF = "fix_nph.html">nph</A> - constant NPH time integration via Nose/Hoover
<LI><A HREF = "fix_npt.html">npt</A> - constant NPT time integration via Nose/Hoover
<LI><A HREF = "fix_npt_asphere.html">npt/asphere</A> - NPT for aspherical particles
<LI><A HREF = "fix_nve.html">nve</A> - constant NVE time integration
<LI><A HREF = "fix_nve_asphere.html">nve/asphere</A> - NVT for aspherical particles
<LI><A HREF = "fix_nve_dipole.html">nve/dipole</A> - NVE for point dipolar particles
<LI><A HREF = "fix_nve_gran.html">nve/gran</A> - NVE for granular particles
<LI><A HREF = "fix_nve_limit.html">nve/limit</A> - NVE with limited step length
<LI><A HREF = "fix_nve_noforce.html">nve/noforce</A> - NVE without forces (v only)
<LI><A HREF = "fix_nvt.html">nvt</A> - constant NVT time integration via Nose/Hoover
<LI><A HREF = "fix_nvt_asphere.html">nvt/asphere</A> - NVT for aspherical particles
<LI><A HREF = "fix_nvt_sllod.html">nvt/sllod</A> - NVT for NEMD with SLLOD equations
<LI><A HREF = "fix_orient_fcc.html">orient/fcc</A> - add grain boundary migration force
<LI><A HREF = "fix_planeforce.html">planeforce</A> - constrain atoms to move in a plane
<LI><A HREF = "fix_poems.html">poems</A> - constrain clusters of atoms to move as coupled rigid bodies
<LI><A HREF = "fix_pour.html">pour</A> - pour new atoms into a granular simulation domain
<LI><A HREF = "fix_print.html">print</A> - print text and variables during a simulation
<LI><A HREF = "fix_rdf.html">rdf</A> - compute radial distribution functions
<LI><A HREF = "fix_recenter.html">recenter</A> - constrain the center-of-mass position of a group of atoms
<LI><A HREF = "fix_rigid.html">rigid</A> - constrain one or more clusters of atoms to move as a rigid body
<LI><A HREF = "fix_setforce.html">setforce</A> - set the force on each atom
<LI><A HREF = "fix_shake.html">shake</A> - SHAKE constraints on bonds and/or angles
<LI><A HREF = "fix_spring.html">spring</A> - apply harmonic spring force to group of atoms
<LI><A HREF = "fix_spring_rg.html">spring/rg</A> - spring on radius of gyration of group of atoms
<LI><A HREF = "fix_spring_self.html">spring/self</A> - spring from each atom to its origin
<LI><A HREF = "fix_temp_rescale.html">temp/rescale</A> - temperature control by velocity rescaling
<LI><A HREF = "fix_tmd.html">tmd</A> - guide a group of atoms to a new configuration
<LI><A HREF = "fix_viscosity.html">viscosity</A> - Muller-Plathe momentum exchange for viscosity calculation
<LI><A HREF = "fix_viscous.html">viscous</A> - viscous damping for granular simulations
<LI><A HREF = "fix_wall_gran.html">wall/gran</A> - frictional wall(s) for granular simulations
<LI><A HREF = "fix_wall_lj126.html">wall/lj126</A> - Lennard-Jones 12-6 wall
<LI><A HREF = "fix_wall_lj93.html">wall/lj93</A> - Lennard-Jones 9-3 wall
<LI><A HREF = "fix_wall_reflect.html">wall/reflect</A> - reflecting wall(s)
<LI><A HREF = "fix_wiggle.html">wiggle</A> - oscillate walls and frozen atoms
</UL>
<P>There are also additional fix styles submitted by users which are
included in the LAMMPS distribution. The list of these with links to
the individual styles are given in the fix section of <A HREF = "Section_commands.html#3_5">this
page</A>.
</P>
<P><B>Restrictions:</B>
</P>
<P>Some fix styles are part of specific packages. They are only enabled
if LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_3">Making
LAMMPS</A> section for more info on packages. The
doc pages for individual fixes tell if it is part of a package.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "unfix.html">unfix</A>, <A HREF = "fix_modify.html">fix_modify</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>