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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 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">fix addforce</A> - add a force to each atom
<LI><A HREF = "fix_aveforce.html">fix aveforce</A> - add an averaged force to each atom
<LI><A HREF = "fix_ave_atom.html">fix ave/atom</A> - compute per-atom time-averaged quantities
<LI><A HREF = "fix_ave_spatial.html">fix ave/spatial</A> - output per-atom quantities by layer
<LI><A HREF = "fix_ave_time.html">fix ave/time</A> - output time-averaged compute quantities
<LI><A HREF = "fix_com.html">fix com</A> - compute a center-of-mass
<LI><A HREF = "fix_deform.html">fix deform</A> - change the simulation box size/shape
<LI><A HREF = "fix_deposit.html">fix deposit</A> - add new atoms above a surface
<LI><A HREF = "fix_drag.html">fix drag</A> - drag atoms towards a defined coordinate
<LI><A HREF = "fix_efield.html">fix efield</A> - impose electric field on system
<LI><A HREF = "fix_enforce2d.html">fix enforce2d</A> - zero out z-dimension velocity and force
<LI><A HREF = "fix_freeze.html">fix freeze</A> - freeze atoms in a granular simulation
<LI><A HREF = "fix_gran_diag.html">fix gran/diag</A> - compute granular diagnostics
<LI><A HREF = "fix_gravity.html">fix gravity</A> - add gravity to atoms in a granular simulation
<LI><A HREF = "fix_gyration.html">fix gyration</A> - compute radius of gyration
<LI><A HREF = "fix_gyration.html">fix heat</A> - add/subtract momentum-conserving heat
<LI><A HREF = "fix_indent.html">fix indent</A> - impose force due to an indenter
<LI><A HREF = "fix_langevin.html">fix langevin</A> - Langevin temperature control
<LI><A HREF = "fix_lineforce.html">fix lineforce</A> - constrain atoms to move in a line
<LI><A HREF = "fix_msd.html">fix msd</A> - compute mean-squared displacement (i.e. diffusion coefficient)
<LI><A HREF = "fix_momentum.html">fix momentum</A> - zero the linear and/or angular momentum of a group of atoms
<LI><A HREF = "fix_nph.html">fix nph</A> - constant NPH time integration via Nose/Hoover
<LI><A HREF = "fix_npt.html">fix npt</A> - constant NPT time integration via Nose/Hoover
<LI><A HREF = "fix_npt_asphere.html">fix npt/asphere</A> - NPT for aspherical particles
<LI><A HREF = "fix_nve.html">fix nve</A> - constant NVE time integration
<LI><A HREF = "fix_nve_asphere.html">fix nve/asphere</A> - NVT for aspherical particles
<LI><A HREF = "fix_nve_dipole.html">fix nve/dipole</A> - NVE for point dipolar particles
<LI><A HREF = "fix_nve_gran.html">fix nve/gran</A> - NVE for granular particles
<LI><A HREF = "fix_nve_limit.html">fix nve/limit</A> - NVE with limited step length
<LI><A HREF = "fix_nve_noforce.html">fix nve/noforce</A> - NVE without forces (v only)
<LI><A HREF = "fix_nvt.html">fix nvt</A> - constant NVT time integration via Nose/Hoover
<LI><A HREF = "fix_nvt_asphere.html">fix nvt/asphere</A> - NVT for aspherical particles
<LI><A HREF = "fix_nvt_sllod.html">fix nvt/sllod</A> - NVT for NEMD with SLLOD equations
<LI><A HREF = "fix_orient_fcc.html">fix orient/fcc</A> - add grain boundary migration force
<LI><A HREF = "fix_planeforce.html">fix planeforce</A> - constrain atoms to move in a plane
<LI><A HREF = "fix_poems.html">fix poems</A> - constrain clusters of atoms to move as coupled rigid bodies
<LI><A HREF = "fix_pour.html">fix pour</A> - pour new atoms into a granular simulation domain
<LI><A HREF = "fix_print.html">fix print</A> - print text and variables during a simulation
<LI><A HREF = "fix_rdf.html">fix rdf</A> - compute radial distribution functions
<LI><A HREF = "fix_recenter.html">fix recenter</A> - constrain the center-of-mass position of a group of atoms
<LI><A HREF = "fix_rigid.html">fix rigid</A> - constrain one or more clusters of atoms to move as a rigid body
<LI><A HREF = "fix_setforce.html">fix setforce</A> - set the force on each atom
<LI><A HREF = "fix_shake.html">fix shake</A> - SHAKE constraints on bonds and/or angles
<LI><A HREF = "fix_spring.html">fix spring</A> - apply harmonic spring force to group of atoms
<LI><A HREF = "fix_spring_rg.html">fix spring/rg</A> - spring on radius of gyration of group of atoms
<LI><A HREF = "fix_spring_self.html">fix spring/self</A> - spring from each atom to its origin
<LI><A HREF = "fix_temp_rescale.html">fix temp/rescale</A> - temperature control by velocity rescaling
<LI><A HREF = "fix_tmd.html">fix tmd</A> - guide a group of atoms to a new configuration
<LI><A HREF = "fix_viscous.html">fix viscous</A> - viscous damping for granular simulations
<LI><A HREF = "fix_wall_gran.html">fix wall/gran</A> - frictional wall(s) for granular simulations
<LI><A HREF = "fix_wall_lj126.html">fix wall/lj126</A> - Lennard-Jones 12-6 wall
<LI><A HREF = "fix_wall_lj93.html">fix wall/lj93</A> - Lennard-Jones 9-3 wall
<LI><A HREF = "fix_wall_reflect.html">fix wall/reflect</A> - reflecting wall(s)
<LI><A HREF = "fix_wiggle.html">fix wiggle</A> - oscillate walls and frozen atoms
</UL>
<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>
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