<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 <aclass="reference internal"href="Section_modify.html"><spanclass="doc">this section</span></a> for a discussion.</p>
<p>Fixes perform their operations at different stages of the timestep.
If 2 or more fixes operate at the same stage of the timestep, they are
invoked in the order they were specified in the input script.</p>
<p>The ID of a fix can only contain alphanumeric characters and
underscores.</p>
<p>Fixes can be deleted with the <aclass="reference internal"href="unfix.html"><spanclass="doc">unfix</span></a> command.</p>
<divclass="admonition note">
<pclass="first admonition-title">Note</p>
<pclass="last">The <aclass="reference internal"href="unfix.html"><spanclass="doc">unfix</span></a> command 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. This is especially important to realize for
<td>one element of vector, one column of array</td>
</tr>
<trclass="row-odd"><td>f_ID[I][J]</td>
<td>one element of array</td>
</tr>
</tbody>
</table>
<p>In other words, using one bracket reduces the dimension of the
quantity once (vector -> scalar, array -> vector). Using two brackets
reduces the dimension twice (array -> scalar). Thus a command that
uses scalar fix values as input can also process elements of a vector
or array.</p>
<p>Note that commands and <aclass="reference internal"href="variable.html"><spanclass="doc">variables</span></a> which use fix
quantities typically do not allow for all kinds, e.g. a command may
require a vector of values, not a scalar. This means there is no
ambiguity about referring to a fix quantity as f_ID even if it
produces, for example, both a scalar and vector. The doc pages for
various commands explain the details.</p>
<hrclass="docutils"/>
<p>In LAMMPS, the values generated by a fix can be used in several ways:</p>
<ulclass="simple">
<li>Global values can be output via the <aclass="reference internal"href="thermo_style.html"><spanclass="doc">thermo_style custom</span></a> or <aclass="reference internal"href="fix_ave_time.html"><spanclass="doc">fix ave/time</span></a> command.
Or the values can be referenced in a <aclass="reference internal"href="variable.html"><spanclass="doc">variable equal</span></a> or
<li>Per-atom values can be output via the <aclass="reference internal"href="dump.html"><spanclass="doc">dump custom</span></a> command.
Or they can be time-averaged via the <aclass="reference internal"href="fix_ave_atom.html"><spanclass="doc">fix ave/atom</span></a>
command or reduced by the <aclass="reference internal"href="compute_reduce.html"><spanclass="doc">compute reduce</span></a>
command. Or the per-atom values can be referenced in an <aclass="reference internal"href="variable.html"><spanclass="doc">atom-style variable</span></a>.</li>
<li>Local values can be reduced by the <aclass="reference internal"href="compute_reduce.html"><spanclass="doc">compute reduce</span></a> command, or histogrammed by the <aclass="reference internal"href="fix_ave_histo.html"><spanclass="doc">fix ave/histo</span></a> command.</li>
</ul>
<p>See this <aclass="reference internal"href="Section_howto.html#howto-15"><spanclass="std std-ref">howto section</span></a> for a summary of
various LAMMPS output options, many of which involve fixes.</p>
<p>The results of fixes that calculate global quantities can be either
“intensive” or “extensive” values. Intensive means the value is
independent of the number of atoms in the simulation,
e.g. temperature. Extensive means the value scales with the number of
atoms in the simulation, e.g. total rotational kinetic energy.
<aclass="reference internal"href="thermo_style.html"><spanclass="doc">Thermodynamic output</span></a> will normalize extensive
values by the number of atoms in the system, depending on the
“thermo_modify norm” setting. It will not normalize intensive values.
If a fix value is accessed in another way, e.g. by a
<aclass="reference internal"href="variable.html"><spanclass="doc">variable</span></a>, you may want to know whether it is an
intensive or extensive value. See the doc page for individual fixes
for further info.</p>
<hrclass="docutils"/>
<p>Each fix style has its own documentation page which describes its
arguments and what it does, as listed below. Here is an alphabetic
list of fix styles available in LAMMPS. They are also given in more
compact form in the Fix section of <aclass="reference internal"href="Section_commands.html#cmd-5"><spanclass="std std-ref">this page</span></a>.</p>
<p>There are also additional fix styles (not listed here) 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 <aclass="reference internal"href="Section_commands.html#cmd-5"><spanclass="std std-ref">this page</span></a>.</p>
<ulclass="simple">
<li><aclass="reference internal"href="fix_adapt.html"><spanclass="doc">adapt</span></a> - change a simulation parameter over time</li>
<li><aclass="reference internal"href="fix_addforce.html"><spanclass="doc">addforce</span></a> - add a force to each atom</li>
<li><aclass="reference internal"href="fix_append_atoms.html"><spanclass="doc">append/atoms</span></a> - append atoms to a running simulation</li>
<li><aclass="reference internal"href="fix_atom_swap.html"><spanclass="doc">atom/swap</span></a> - Monte Carlo atom type swapping</li>
<li><aclass="reference internal"href="fix_aveforce.html"><spanclass="doc">aveforce</span></a> - add an averaged force to each atom</li>
<li><aclass="reference internal"href="fix_indent.html"><spanclass="doc">indent</span></a> - impose force due to an indenter</li>
<li><aclass="reference internal"href="fix_langevin.html"><spanclass="doc">langevin</span></a> - Langevin temperature control</li>
<li><aclass="reference internal"href="fix_lineforce.html"><spanclass="doc">lineforce</span></a> - constrain atoms to move in a line</li>
<li><aclass="reference internal"href="fix_momentum.html"><spanclass="doc">momentum</span></a> - zero the linear and/or angular momentum of a group of atoms</li>
<li><aclass="reference internal"href="fix_move.html"><spanclass="doc">move</span></a> - move atoms in a prescribed fashion</li>
<li><aclass="reference internal"href="fix_nve_asphere.html"><spanclass="doc">nve/asphere</span></a> - NVE for aspherical particles</li>
<li><aclass="reference internal"href="fix_nve_asphere_noforce.html"><spanclass="doc">nve/asphere/noforce</span></a> - NVE for aspherical particles without forces”</li>
<li><aclass="reference internal"href="fix_nve_body.html"><spanclass="doc">nve/body</span></a> - NVE for body particles</li>
<li><aclass="reference internal"href="fix_nve_limit.html"><spanclass="doc">nve/limit</span></a> - NVE with limited step length</li>
<li><aclass="reference internal"href="fix_nve_line.html"><spanclass="doc">nve/line</span></a> - NVE for line segments</li>
<li><aclass="reference internal"href="fix_nve_noforce.html"><spanclass="doc">nve/noforce</span></a> - NVE without forces (v only)</li>
<li><aclass="reference internal"href="fix_nve_sphere.html"><spanclass="doc">nve/sphere</span></a> - NVE for spherical particles</li>
<li><aclass="reference internal"href="fix_nve_tri.html"><spanclass="doc">nve/tri</span></a> - NVE for triangles</li>
<li><aclass="reference internal"href="fix_nh.html"><spanclass="doc">nvt</span></a> - constant NVT time integration via Nose/Hoover</li>
<li><aclass="reference internal"href="fix_nvt_asphere.html"><spanclass="doc">nvt/asphere</span></a> - NVT for aspherical particles</li>
<li><aclass="reference internal"href="fix_nve_body.html"><spanclass="doc">nvt/body</span></a> - NVT for body particles</li>
<li><aclass="reference internal"href="fix_nvt_sllod.html"><spanclass="doc">nvt/sllod</span></a> - NVT for NEMD with SLLOD equations</li>
<li><aclass="reference internal"href="fix_nvt_sphere.html"><spanclass="doc">nvt/sphere</span></a> - NVT for spherical particles</li>
<li><aclass="reference internal"href="fix_oneway.html"><spanclass="doc">oneway</span></a> - constrain particles on move in one direction</li>
<li><aclass="reference internal"href="fix_planeforce.html"><spanclass="doc">planeforce</span></a> - constrain atoms to move in a plane</li>
<li><aclass="reference internal"href="fix_poems.html"><spanclass="doc">poems</span></a> - constrain clusters of atoms to move as coupled rigid bodies</li>
<li><aclass="reference internal"href="fix_pour.html"><spanclass="doc">pour</span></a> - pour new atoms/molecules into a granular simulation domain</li>
<li><aclass="reference internal"href="fix_press_berendsen.html"><spanclass="doc">press/berendsen</span></a> - pressure control by Berendsen barostat</li>
<li><aclass="reference internal"href="fix_print.html"><spanclass="doc">print</span></a> - print text and variables during a simulation</li>
<li><aclass="reference internal"href="fix_qeq_comb.html"><spanclass="doc">qeq/comb</span></a> - charge equilibration for COMB potential <aclass="reference internal"href="fix_qeq.html"><spanclass="doc">qeq/dynamic</span></a> - charge equilibration via dynamic method <aclass="reference internal"href="fix_qeq.html"><spanclass="doc">qeq/fire</span></a> - charge equilibration via FIRE minimizer <aclass="reference internal"href="fix_qeq.html"><spanclass="doc">qeq/point</span></a> - charge equilibration via point method <aclass="reference internal"href="fix_qeq.html"><spanclass="doc">qeq/shielded</span></a> - charge equilibration via shielded method <aclass="reference internal"href="fix_qeq.html"><spanclass="doc">qeq/slater</span></a> - charge equilibration via Slater method <aclass="reference internal"href="fix_shake.html"><spanclass="doc">rattle</span></a> - RATTLE constraints on bonds and/or angles</li>
<li><aclass="reference internal"href="fix_reax_bonds.html"><spanclass="doc">reax/bonds</span></a> - write out ReaxFF bond information <aclass="reference internal"href="fix_recenter.html"><spanclass="doc">recenter</span></a> - constrain the center-of-mass position of a group of atoms</li>
<li><aclass="reference internal"href="fix_restrain.html"><spanclass="doc">restrain</span></a> - constrain a bond, angle, dihedral</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid</span></a> - constrain one or more clusters of atoms to move as a rigid body with NVE integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/nph</span></a> - constrain one or more clusters of atoms to move as a rigid body with NPH integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/npt</span></a> - constrain one or more clusters of atoms to move as a rigid body with NPT integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/nve</span></a> - constrain one or more clusters of atoms to move as a rigid body with alternate NVE integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/nvt</span></a> - constrain one or more clusters of atoms to move as a rigid body with NVT integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/small</span></a> - constrain many small clusters of atoms to move as a rigid body with NVE integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/small/nph</span></a> - constrain many small clusters of atoms to move as a rigid body with NPH integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/small/npt</span></a> - constrain many small clusters of atoms to move as a rigid body with NPT integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/small/nve</span></a> - constrain many small clusters of atoms to move as a rigid body with alternate NVE integration</li>
<li><aclass="reference internal"href="fix_rigid.html"><spanclass="doc">rigid/small/nvt</span></a> - constrain many small clusters of atoms to move as a rigid body with NVT integration</li>
<li><aclass="reference internal"href="fix_setforce.html"><spanclass="doc">setforce</span></a> - set the force on each atom</li>
<li><aclass="reference internal"href="fix_shake.html"><spanclass="doc">shake</span></a> - SHAKE constraints on bonds and/or angles</li>
<li><aclass="reference internal"href="fix_spring.html"><spanclass="doc">spring</span></a> - apply harmonic spring force to group of atoms</li>
<li><aclass="reference internal"href="fix_spring_chunk.html"><spanclass="doc">spring/chunk</span></a> - apply harmonic spring force to each chunk of atoms</li>
<li><aclass="reference internal"href="fix_store_force.html"><spanclass="doc">store/force</span></a> - store force on each atom</li>
<li><aclass="reference internal"href="fix_store_state.html"><spanclass="doc">store/state</span></a> - store attributes for each atom</li>
<li><aclass="reference internal"href="fix_temp_berendsen.html"><spanclass="doc">temp/berendsen</span></a> - temperature control by Berendsen thermostat</li>
<li><aclass="reference internal"href="fix_temp_csvr.html"><spanclass="doc">temp/csld</span></a> - canonical sampling thermostat with Langevin dynamics</li>
<li><aclass="reference internal"href="fix_temp_csvr.html"><spanclass="doc">temp/csvr</span></a> - canonical sampling thermostat with Hamiltonian dynamics</li>
<li><aclass="reference internal"href="fix_temp_rescale.html"><spanclass="doc">temp/rescale</span></a> - temperature control by velocity rescaling</li>
<li><aclass="reference internal"href="fix_tfmc.html"><spanclass="doc">tfmc</span></a> - perform force-bias Monte Carlo with time-stamped method</li>
<li><aclass="reference internal"href="fix_thermal_conductivity.html"><spanclass="doc">thermal/conductivity</span></a> - Muller-Plathe kinetic energy exchange for thermal conductivity calculation</li>
<li><aclass="reference internal"href="fix_tmd.html"><spanclass="doc">tmd</span></a> - guide a group of atoms to a new configuration</li>
<li><aclass="reference internal"href="fix_ttm.html"><spanclass="doc">ttm</span></a> - two-temperature model for electronic/atomic coupling</li>
<li><aclass="reference internal"href="fix_wall_region.html"><spanclass="doc">wall/region</span></a> - use region surface as wall</li>
<li><aclass="reference internal"href="fix_wall_srd.html"><spanclass="doc">wall/srd</span></a> - slip/no-slip wall for SRD particles</li>
</ul>
</div>
<divclass="section"id="restrictions">
<h2>Restrictions</h2>
<p>Some fix styles are part of specific packages. They are only enabled
if LAMMPS was built with that package. See the <aclass="reference internal"href="Section_start.html#start-3"><spanclass="std std-ref">Making LAMMPS</span></a> section for more info on packages.
The doc pages for individual fixes tell if it is part of a package.</p>
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