2006-09-22 00:22:34 +08:00
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"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix command :h3
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[Syntax:]
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fix ID group-ID style args :pre
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ID = user-assigned name for the fix
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group-ID = ID of the group of atoms to apply the fix to
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style = one of a long list of possible style names (see below)
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args = arguments used by a particular style :ul
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[Examples:]
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fix 1 all nve
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fix 3 all nvt 300.0 300.0 0.01
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fix mine top setforce 0.0 NULL 0.0 :pre
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[Description:]
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Set a fix that will be applied to a group of atoms. In LAMMPS, a
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"fix" is any operation that is applied to the system during
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timestepping or minimization. Examples include updating of atom
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positions and velocities due to time integration, controlling
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temperature, applying constraint forces to atoms, enforcing boundary
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conditions, computing diagnostics, etc. There are dozens of fixes
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defined in LAMMPS and new ones can be added - see "this
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section"_Section_modify.html for a discussion.
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Each fix style has its own documentation page which describes its
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2007-06-26 08:03:39 +08:00
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arguments and what it does, as listed below.
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2006-09-22 00:22:34 +08:00
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Fixes perform their operations at different stages of the timestep.
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If 2 or more fixes both operate at the same stage of the timestep,
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they are invoked in the order they were specified in the input script.
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Fixes can be deleted with the "unfix"_unfix.html command. Note that
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this is the only way to turn off a fix; simply specifying a new fix
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with a similar style will not turn off the first one. For example,
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2007-06-26 08:03:39 +08:00
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using a "fix nve"_fix_nve.html command for a second run after using a
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"fix nvt"_fix_nvt.html command for the first run, will not cancel out
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the NVT time integration invoked by the "fix nvt" command. Thus two
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time integrators would be in place!
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2006-09-22 00:22:34 +08:00
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2006-12-16 06:53:57 +08:00
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If you specify a new fix with the same ID and style as an existing
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fix, the old fix is deleted and the new one is created (presumably
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with new settings). This is the same as if an "unfix" command were
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first performed on the old fix, except that the new fix is kept in the
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same order relative to the existing fixes as the old one originally
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was. Note that this operation also wipes out any additional changes
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made to the old fix via the "fix_modify"_fix_modify.html command.
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2007-06-26 08:03:39 +08:00
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The "fix modify"_fix_modify.html command allows settings for some
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fixes to be reset. See the doc page for individual fixes for details.
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Some fixes store an internal "state" which is written to binary
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restart files via the "restart"_restart.html or
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"write_restart"_write_restart.html commands. This allows the fix to
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continue on with its calculations in a restarted simulation. See the
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"read_restart"_read_restart.html command for info on how to re-specify
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a fix in an input script that reads a restart file. See the doc pages
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for individual fixes for info on which ones can be restarted.
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Here is an alphabetic list of fix styles available in LAMMPS:
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2006-09-22 00:22:34 +08:00
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"fix addforce"_fix_addforce.html - add a force to each atom
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"fix aveforce"_fix_aveforce.html - add an averaged force to each atom
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2007-09-28 07:25:52 +08:00
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"fix ave/atom"_fix_ave_atom.html - compute per-atom time-averaged quantities
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2007-02-21 08:18:01 +08:00
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"fix ave/spatial"_fix_ave_spatial.html - output per-atom quantities by layer
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"fix ave/time"_fix_ave_time.html - output time-averaged compute quantities
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"fix com"_fix_com.html - compute a center-of-mass
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"fix deform"_fix_deform.html - change the simulation box size/shape
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"fix deposit"_fix_deposit.html - add new atoms above a surface
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"fix drag"_fix_drag.html - drag atoms towards a defined coordinate
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"fix efield"_fix_efield.html - impose electric field on system
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"fix enforce2d"_fix_enforce2d.html - zero out z-dimension velocity and force
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"fix freeze"_fix_freeze.html - freeze atoms in a granular simulation
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"fix gran/diag"_fix_gran_diag.html - compute granular diagnostics
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"fix gravity"_fix_gravity.html - add gravity to atoms in a granular simulation
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"fix gyration"_fix_gyration.html - compute radius of gyration
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2007-10-03 05:55:02 +08:00
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"fix heat"_fix_gyration.html - add/subtract momentum-conserving heat
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"fix indent"_fix_indent.html - impose force due to an indenter
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"fix langevin"_fix_langevin.html - Langevin temperature control
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"fix lineforce"_fix_lineforce.html - constrain atoms to move in a line
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"fix msd"_fix_msd.html - compute mean-squared displacement \
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(i.e. diffusion coefficient)
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"fix momentum"_fix_momentum.html - zero the linear and/or angular momentum \
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of a group of atoms
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"fix nph"_fix_nph.html - constant NPH time integration via Nose/Hoover
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"fix npt"_fix_npt.html - constant NPT time integration via Nose/Hoover
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"fix npt/asphere"_fix_npt_asphere.html - NPT for aspherical particles
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"fix nve"_fix_nve.html - constant NVE time integration
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"fix nve/asphere"_fix_nve_asphere.html - NVT for aspherical particles
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"fix nve/dipole"_fix_nve_dipole.html - NVE for point dipolar particles
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"fix nve/gran"_fix_nve_gran.html - NVE for granular particles
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"fix nve/limit"_fix_nve_limit.html - NVE with limited step length
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"fix nve/noforce"_fix_nve_noforce.html - NVE without forces (v only)
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"fix nvt"_fix_nvt.html - constant NVT time integration via Nose/Hoover
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"fix nvt/asphere"_fix_nvt_asphere.html - NVT for aspherical particles
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"fix nvt/sllod"_fix_nvt_sllod.html - NVT for NEMD with SLLOD equations
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"fix orient/fcc"_fix_orient_fcc.html - add grain boundary migration force
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"fix planeforce"_fix_planeforce.html - constrain atoms to move in a plane
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"fix poems"_fix_poems.html - constrain clusters of atoms to move \
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as coupled rigid bodies
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2006-12-13 08:37:25 +08:00
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"fix pour"_fix_pour.html - pour new atoms into a granular simulation domain
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"fix print"_fix_print.html - print text and variables during a simulation
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"fix rdf"_fix_rdf.html - compute radial distribution functions
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"fix recenter"_fix_recenter.html - constrain the center-of-mass position \
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of a group of atoms
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"fix rigid"_fix_rigid.html - constrain one or more clusters of atoms to \
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move as a rigid body
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"fix setforce"_fix_setforce.html - set the force on each atom
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"fix shake"_fix_shake.html - SHAKE constraints on bonds and/or angles
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"fix spring"_fix_spring.html - apply harmonic spring force to group of atoms
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"fix spring/rg"_fix_spring_rg.html - spring on radius of gyration of \
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group of atoms
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"fix spring/self"_fix_spring_self.html - spring from each atom to its origin
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"fix temp/rescale"_fix_temp_rescale.html - temperature control by \
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velocity rescaling
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"fix tmd"_fix_tmd.html - guide a group of atoms to a new configuration
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"fix viscous"_fix_viscous.html - viscous damping for granular simulations
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"fix wall/gran"_fix_wall_gran.html - frictional wall(s) for \
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granular simulations
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2006-11-11 08:08:49 +08:00
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"fix wall/lj126"_fix_wall_lj126.html - Lennard-Jones 12-6 wall
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"fix wall/lj93"_fix_wall_lj93.html - Lennard-Jones 9-3 wall
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"fix wall/reflect"_fix_wall_reflect.html - reflecting wall(s)
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"fix wiggle"_fix_wiggle.html - oscillate walls and frozen atoms :ul
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2007-10-03 06:56:45 +08:00
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There are also additional fix styles submitted by users which are
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included in the LAMMPS distribution. The list of these with links to
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the individual styles are given in the fix section of "this
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page"_Section_commands.html#3_5.
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2006-09-22 00:22:34 +08:00
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[Restrictions:]
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Some fix styles are part of specific packages. They are only enabled
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if LAMMPS was built with that package. See the "Making
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LAMMPS"_Section_start.html#2_3 section for more info on packages. The
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doc pages for individual fixes tell if it is part of a package.
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[Related commands:]
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"unfix"_unfix.html, "fix_modify"_fix_modify.html
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[Default:] none
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