<li>ID, group-ID are documented in <aclass="reference internal"href="fix.html"><spanclass="doc">fix</span></a> command</li>
<li>style_name = <em>nvt</em> or <em>npt</em> or <em>nph</em></li>
<li>one or more keyword/value pairs may be appended</li>
</ul>
<preclass="literal-block">
keyword = <em>temp</em> or <em>iso</em> or <em>aniso</em> or <em>tri</em> or <em>x</em> or <em>y</em> or <em>z</em> or <em>xy</em> or <em>yz</em> or <em>xz</em> or <em>couple</em> or <em>tchain</em> or <em>pchain</em> or <em>mtk</em> or <em>tloop</em> or <em>ploop</em> or <em>nreset</em> or <em>drag</em> or <em>dilate</em> or <em>scalexy</em> or <em>scaleyz</em> or <em>scalexz</em> or <em>flip</em> or <em>fixedpoint</em> or <em>update</em>
<em>temp</em> values = Tstart Tstop Tdamp
Tstart,Tstop = external temperature at start/end of run
Tdamp = temperature damping parameter (time units)
<em>iso</em> or <em>aniso</em> or <em>tri</em> values = Pstart Pstop Pdamp
Pstart,Pstop = scalar external pressure at start/end of run (pressure units)
Pdamp = pressure damping parameter (time units)
<em>x</em> or <em>y</em> or <em>z</em> or <em>xy</em> or <em>yz</em> or <em>xz</em> values = Pstart Pstop Pdamp
Pstart,Pstop = external stress tensor component at start/end of run (pressure units)
Pdamp = stress damping parameter (time units)
<em>couple</em> = <em>none</em> or <em>xyz</em> or <em>xy</em> or <em>yz</em> or <em>xz</em>
<em>tchain</em> value = N
N = length of thermostat chain (1 = single thermostat)
<em>pchain</em> values = N
N length of thermostat chain on barostat (0 = no thermostat)
<em>mtk</em> value = <em>yes</em> or <em>no</em> = add in MTK adjustment term or not
<em>tloop</em> value = M
M = number of sub-cycles to perform on thermostat
<em>ploop</em> value = M
M = number of sub-cycles to perform on barostat thermostat
<em>nreset</em> value = reset reference cell every this many timesteps
<em>drag</em> value = Df
Df = drag factor added to barostat/thermostat (0.0 = no drag)
<em>dilate</em> value = dilate-group-ID
dilate-group-ID = only dilate atoms in this group due to barostat volume changes
<em>scalexy</em> value = <em>yes</em> or <em>no</em> = scale xy with ly
<em>scaleyz</em> value = <em>yes</em> or <em>no</em> = scale yz with lz
<em>scalexz</em> value = <em>yes</em> or <em>no</em> = scale xz with lz
<em>flip</em> value = <em>yes</em> or <em>no</em> = allow or disallow box flips when it becomes highly skewed
<em>fixedpoint</em> values = x y z
x,y,z = perform barostat dilation/contraction around this point (distance units)
<p>The barostat parameters for fix styles <em>npt</em> and <em>nph</em> is specified
using one or more of the <em>iso</em>, <em>aniso</em>, <em>tri</em>, <em>x</em>, <em>y</em>, <em>z</em>, <em>xy</em>,
<em>xz</em>, <em>yz</em>, and <em>couple</em> keywords. These keywords give you the
ability to specify all 6 components of an external stress tensor, and
to couple various of these components together so that the dimensions
they represent are varied together during a constant-pressure
simulation.</p>
<p>Other barostat-related keywords are <em>pchain</em>, <em>mtk</em>, <em>ploop</em>,
<em>nreset</em>, <em>drag</em>, and <em>dilate</em>, which are discussed below.</p>
<p>Orthogonal simulation boxes have 3 adjustable dimensions (x,y,z).
Triclinic (non-orthogonal) simulation boxes have 6 adjustable
dimensions (x,y,z,xy,xz,yz). The <aclass="reference internal"href="create_box.html"><spanclass="doc">create_box</span></a>, <aclass="reference internal"href="read_data.html"><spanclass="doc">read data</span></a>, and <aclass="reference internal"href="read_restart.html"><spanclass="doc">read_restart</span></a> commands
specify whether the simulation box is orthogonal or non-orthogonal
(triclinic) and explain the meaning of the xy,xz,yz tilt factors.</p>
<p>The target pressures for each of the 6 components of the stress tensor
can be specified independently via the <em>x</em>, <em>y</em>, <em>z</em>, <em>xy</em>, <em>xz</em>, <em>yz</em>
keywords, which correspond to the 6 simulation box dimensions. For
each component, the external pressure or tensor component at each
timestep is a ramped value during the run from <em>Pstart</em> to <em>Pstop</em>.
If a target pressure is specified for a component, then the
corresponding box dimension will change during a simulation. For
example, if the <em>y</em> keyword is used, the y-box length will change. If
the <em>xy</em> keyword is used, the xy tilt factor will change. A box
dimension will not change if that component is not specified, although
you have the option to change that dimension via the <aclass="reference internal"href="fix_deform.html"><spanclass="doc">fix deform</span></a> command.</p>
<p>Note that in order to use the <em>xy</em>, <em>xz</em>, or <em>yz</em> keywords, the
simulation box must be triclinic, even if its initial tilt factors are
0.0.</p>
<p>For all barostat keywords, the <em>Pdamp</em> parameter operates like the
<em>Tdamp</em> parameter, determining the time scale on which pressure is
relaxed. For example, a value of 10.0 means to relax the pressure in
a timespan of (roughly) 10 time units (e.g. tau or fmsec or psec - see
does the same thing for the barostat thermostat.</p>
<p>The keyword <em>nreset</em> controls how often the reference dimensions used
to define the strain energy are reset. If this keyword is not used,
or is given a value of zero, then the reference dimensions are set to
those of the initial simulation domain and are never changed. If the
simulation domain changes significantly during the simulation, then
the final average pressure tensor will differ significantly from the
specified values of the external stress tensor. A value of <em>nstep</em>
means that every <em>nstep</em> timesteps, the reference dimensions are set
to those of the current simulation domain.</p>
<p>The <em>scaleyz</em>, <em>scalexz</em>, and <em>scalexy</em> keywords control whether or
not the corresponding tilt factors are scaled with the associated box
dimensions when barostatting triclinic periodic cells. The default
values <em>yes</em> will turn on scaling, which corresponds to adjusting the
linear dimensions of the cell while preserving its shape. Choosing
<em>no</em> ensures that the tilt factors are not scaled with the box
dimensions. See below for restrictions and default values in different
situations. In older versions of LAMMPS, scaling of tilt factors was
not performed. The old behavior can be recovered by setting all three
scale keywords to <em>no</em>.</p>
<p>The <em>flip</em> keyword allows the tilt factors for a triclinic box to
exceed half the distance of the parallel box length, as discussed
below. If the <em>flip</em> value is set to <em>yes</em>, the bound is enforced by
flipping the box when it is exceeded. If the <em>flip</em> value is set to
<em>no</em>, the tilt will continue to change without flipping. Note that if
applied stress induces large deformations (e.g. in a liquid), this
means the box shape can tilt dramatically and LAMMPS will run less
efficiently, due to the large volume of communication needed to
acquire ghost atoms around a processor’s irregular-shaped sub-domain.
For extreme values of tilt, LAMMPS may also lose atoms and generate an
error.</p>
<p>The <em>fixedpoint</em> keyword specifies the fixed point for barostat volume
changes. By default, it is the center of the box. Whatever point is
chosen will not move during the simulation. For example, if the lower
periodic boundaries pass through (0,0,0), and this point is provided
to <em>fixedpoint</em>, then the lower periodic boundaries will remain at
(0,0,0), while the upper periodic boundaries will move twice as
far. In all cases, the particle trajectories are unaffected by the
chosen value, except for a time-dependent constant translation of
positions.</p>
<p>If the <em>update</em> keyword is used with the <em>dipole</em> value, then the
orientation of the dipole moment of each particle is also updated
during the time integration. This option should be used for models
where a dipole moment is assigned to finite-size particles,
e.g. spheroids via use of the <aclass="reference internal"href="atom_style.html"><spanclass="doc">atom_style hybrid sphere dipole</span></a> command.</p>
should not use any other time integration fix, such as <aclass="reference internal"href="fix_nve.html"><spanclass="doc">fix nve</span></a> on atoms to which this fix is applied. Likewise,
fix nvt and fix npt should not normally be used on atoms that also
have their temperature controlled by another fix - e.g. by <aclass="reference internal"href="#"><spanclass="doc">fix langevin</span></a> or <aclass="reference internal"href="fix_temp_rescale.html"><spanclass="doc">fix temp/rescale</span></a>
commands.</p>
</div>
<p>See <aclass="reference internal"href="Section_howto.html#howto-16"><spanclass="std std-ref">this howto section</span></a> of the manual for
a discussion of different ways to compute temperature and perform
thermostatting and barostatting.</p>
<hrclass="docutils"/>
<p>These fixes compute a temperature and pressure each timestep. To do
this, the fix creates its own computes of style “temp” and “pressure”,
as if one of these two sets of commands had been issued:</p>
<p>See the <aclass="reference internal"href="compute_temp.html"><spanclass="doc">compute temp</span></a> and <aclass="reference internal"href="compute_pressure.html"><spanclass="doc">compute pressure</span></a> commands for details. Note that the
IDs of the new computes are the fix-ID + underscore + “temp” or fix_ID
+ underscore + “press”. For fix nvt, the group for the new computes
is the same as the fix group. For fix nph and fix npt, the group for
the new computes is “all” since pressure is computed for the entire
system.</p>
<p>Note that these are NOT the computes used by thermodynamic output (see
the <aclass="reference internal"href="thermo_style.html"><spanclass="doc">thermo_style</span></a> command) with ID = <em>thermo_temp</em>
and <em>thermo_press</em>. This means you can change the attributes of this
fix’s temperature or pressure via the
<aclass="reference internal"href="compute_modify.html"><spanclass="doc">compute_modify</span></a> command or print this temperature
or pressure during thermodynamic output via the <aclass="reference internal"href="thermo_style.html"><spanclass="doc">thermo_style custom</span></a> command using the appropriate compute-ID.
It also means that changing attributes of <em>thermo_temp</em> or
<em>thermo_press</em> will have no effect on this fix.</p>
<p>Like other fixes that perform thermostatting, fix nvt and fix npt can
be used with <aclass="reference internal"href="compute.html"><spanclass="doc">compute commands</span></a> that calculate a
temperature after removing a “bias” from the atom velocities.
E.g. removing the center-of-mass velocity from a group of atoms or
only calculating temperature on the x-component of velocity or only
calculating temperature for atoms in a geometric region. This is not
done by default, but only if the <aclass="reference internal"href="fix_modify.html"><spanclass="doc">fix_modify</span></a> command
is used to assign a temperature compute to this fix that includes such
a bias term. See the doc pages for individual <aclass="reference internal"href="compute.html"><spanclass="doc">compute commands</span></a> to determine which ones include a bias. In
this case, the thermostat works in the following manner: the current
temperature is calculated taking the bias into account, bias is
removed from each atom, thermostatting is performed on the remaining
thermal degrees of freedom, and the bias is added back in.</p>
<hrclass="docutils"/>
<p>These fixes can be used with either the <em>verlet</em> or <em>respa</em>
<aclass="reference internal"href="run_style.html"><spanclass="doc">integrators</span></a>. When using one of the barostat fixes
with <em>respa</em>, LAMMPS uses an integrator constructed
according to the following factorization of the Liouville propagator
<p>The fix npt and fix nph commands can be used with rigid bodies or
mixtures of rigid bodies and non-rigid particles (e.g. solvent). But
there are also <aclass="reference internal"href="fix_rigid.html"><spanclass="doc">fix rigid/npt</span></a> and <aclass="reference internal"href="fix_rigid.html"><spanclass="doc">fix rigid/nph</span></a> commands, which are typically a more natural
choice. See the doc page for those commands for more discussion of
<p>These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
USER-OMP and OPT packages, respectively. They are only enabled if
LAMMPS was built with those packages. See the <aclass="reference internal"href="Section_start.html#start-3"><spanclass="std std-ref">Making LAMMPS</span></a> section for more info.</p>
<p>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <aclass="reference internal"href="Section_start.html#start-7"><spanclass="std std-ref">-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
use the <aclass="reference internal"href="suffix.html"><spanclass="doc">suffix</span></a> command in your input script.</p>
<p>See <aclass="reference internal"href="Section_accelerate.html"><spanclass="doc">Section_accelerate</span></a> of the manual for
more instructions on how to use the accelerated styles effectively.</p>
<p>These fixes writes the state of all the thermostat and barostat
variables to <aclass="reference internal"href="restart.html"><spanclass="doc">binary restart files</span></a>. See the
<aclass="reference internal"href="read_restart.html"><spanclass="doc">read_restart</span></a> command for info on how to re-specify
a fix in an input script that reads a restart file, so that the
operation of the fix continues in an uninterrupted fashion.</p>
<p>The <aclass="reference internal"href="fix_modify.html"><spanclass="doc">fix_modify</span></a><em>temp</em> and <em>press</em> options are
supported by these fixes. You can use them to assign a
<aclass="reference internal"href="compute.html"><spanclass="doc">compute</span></a> you have defined to this fix which will be used
in its thermostatting or barostatting procedure, as described above.
If you do this, note that the kinetic energy derived from the compute
temperature should be consistent with the virial term computed using
all atoms for the pressure. LAMMPS will warn you if you choose to
compute temperature on a subset of atoms.</p>
<divclass="admonition note">
<pclass="first admonition-title">Note</p>
<pclass="last">If both the <em>temp</em> and <em>press</em> keywords are used in a single
thermo_modify command (or in two separate commands), then the order in
which the keywords are specified is important. Note that a <aclass="reference internal"href="compute_pressure.html"><spanclass="doc">pressure compute</span></a> defines its own temperature compute as
an argument when it is specified. The <em>temp</em> keyword will override
this (for the pressure compute being used by fix npt), but only if the
<em>temp</em> keyword comes after the <em>press</em> keyword. If the <em>temp</em> keyword
comes before the <em>press</em> keyword, then the new pressure compute
specified by the <em>press</em> keyword will be unaffected by the <em>temp</em>
setting.</p>
</div>
<p>The <aclass="reference internal"href="fix_modify.html"><spanclass="doc">fix_modify</span></a><em>energy</em> option is supported by these
fixes to add the energy change induced by Nose/Hoover thermostatting
and barostatting to the system’s potential energy as part of
<p>These fixes compute a global scalar and a global vector of quantities,
which can be accessed by various <aclass="reference internal"href="Section_howto.html#howto-15"><spanclass="std std-ref">output commands</span></a>. The scalar value calculated by
these fixes is “extensive”; the vector values are “intensive”.</p>
<p>The scalar is the cumulative energy change due to the fix.</p>
<p>The vector stores internal Nose/Hoover thermostat and barostat
variables. The number and meaning of the vector values depends on
which fix is used and the settings for keywords <em>tchain</em> and <em>pchain</em>,
which specify the number of Nose/Hoover chains for the thermostat and
barostat. If no thermostatting is done, then <em>tchain</em> is 0. If no
barostatting is done, then <em>pchain</em> is 0. In the following list,
“ndof” is 0, 1, 3, or 6, and is the number of degrees of freedom in
the barostat. Its value is 0 if no barostat is used, else its value
is 6 if any off-diagonal stress tensor component is barostatted, else
its value is 1 if <em>couple xyz</em> is used or <em>couple xy</em> for a 2d
simulation, otherwise its value is 3.</p>
<p>The order of values in the global vector and their meaning is as
follows. The notation means there are tchain values for eta, followed
by tchain for eta_dot, followed by ndof for omega, etc:</p>
<li>PE_eta[tchain] = potential energy of each particle thermostat displacement (energy units)</li>
<li>KE_eta_dot[tchain] = kinetic energy of each particle thermostat velocity (energy units)</li>
<li>PE_omega[ndof] = potential energy of each barostat displacement (energy units)</li>
<li>KE_omega_dot[ndof] = kinetic energy of each barostat velocity (energy units)</li>
<li>PE_etap[pchain] = potential energy of each barostat thermostat displacement (energy units)</li>
<li>KE_etap_dot[pchain] = kinetic energy of each barostat thermostat velocity (energy units)</li>
<li>PE_strain[1] = scalar strain energy (energy units)</li>
</ul>
<p>These fixes can ramp their external temperature and pressure over
multiple runs, using the <em>start</em> and <em>stop</em> keywords of the
<aclass="reference internal"href="run.html"><spanclass="doc">run</span></a> command. See the <aclass="reference internal"href="run.html"><spanclass="doc">run</span></a> command for details of
how to do this.</p>
<p>These fixes are not invoked during <aclass="reference internal"href="minimize.html"><spanclass="doc">energy minimization</span></a>.</p>
</div>
<hrclass="docutils"/>
<divclass="section"id="restrictions">
<h2>Restrictions</h2>
<p><em>X</em>, <em>y</em>, <em>z</em> cannot be barostatted if the associated dimension is not
periodic. <em>Xy</em>, <em>xz</em>, and <em>yz</em> can only be barostatted if the
simulation domain is triclinic and the 2nd dimension in the keyword
(<em>y</em> dimension in <em>xy</em>) is periodic. <em>Z</em>, <em>xz</em>, and <em>yz</em>, cannot be
barostatted for 2D simulations. The <aclass="reference internal"href="create_box.html"><spanclass="doc">create_box</span></a>,
<aclass="reference internal"href="read_data.html"><spanclass="doc">read data</span></a>, and <aclass="reference internal"href="read_restart.html"><spanclass="doc">read_restart</span></a>
commands specify whether the simulation box is orthogonal or
non-orthogonal (triclinic) and explain the meaning of the xy,xz,yz
tilt factors.</p>
<p>For the <em>temp</em> keyword, the final Tstop cannot be 0.0 since it would
make the external T = 0.0 at some timestep during the simulation which
is not allowed in the Nose/Hoover formulation.</p>
<p>The <em>scaleyz yes</em> and <em>scalexz yes</em> keyword/value pairs can not be used
for 2D simulations. <em>scaleyz yes</em>, <em>scalexz yes</em>, and <em>scalexy yes</em> options
can only be used if the 2nd dimension in the keyword is periodic,
and if the tilt factor is not coupled to the barostat via keywords
<em>tri</em>, <em>yz</em>, <em>xz</em>, and <em>xy</em>.</p>
<p>These fixes can be used with dynamic groups as defined by the
<aclass="reference internal"href="group.html"><spanclass="doc">group</span></a> command. Likewise they can be used with groups to
which atoms are added or deleted over time, e.g. a deposition
simulation. However, the conservation properties of the thermostat
and barostat are defined for systems with a static set of atoms. You
may observe odd behavior if the atoms in a group vary dramatically
over time or the atom count becomes very small.</p>
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