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<div class="section" id="fix-command">
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<span id="index-0"></span><h1>fix command<a class="headerlink" href="#fix-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID style args
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID = user-assigned name for the fix</li>
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<li>group-ID = ID of the group of atoms to apply the fix to</li>
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<li>style = one of a long list of possible style names (see below)</li>
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<li>args = arguments used by a particular style</li>
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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix 1 all nve
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fix 3 all nvt temp 300.0 300.0 0.01
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fix mine top setforce 0.0 NULL 0.0
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>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 <a class="reference internal" href="Section_modify.html"><em>this section</em></a> for a discussion.</p>
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<p>Fixes perform their operations at different stages of the timestep.
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If 2 or more fixes operate at the same stage of the timestep, they are
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invoked in the order they were specified in the input script.</p>
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<p>The ID of a fix can only contain alphanumeric characters and
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underscores.</p>
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<p>Fixes can be deleted with the <a class="reference internal" href="unfix.html"><em>unfix</em></a> command.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">The <a class="reference internal" href="unfix.html"><em>unfix</em></a> command is the only way to turn off a
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fix; simply specifying a new fix with a similar style will not turn
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off the first one. This is especially important to realize for
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integration fixes. For example, using a <a class="reference internal" href="fix_nve.html"><em>fix nve</em></a>
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command for a second run after using a <a class="reference internal" href="fix_nh.html"><em>fix nvt</em></a> command
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for the first run, will not cancel out the NVT time integration
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invoked by the “fix nvt” command. Thus two time integrators would be
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in place!</p>
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</div>
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<p>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 <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> command.</p>
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<p>The <a class="reference internal" href="fix_modify.html"><em>fix modify</em></a> command allows settings for some
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fixes to be reset. See the doc page for individual fixes for details.</p>
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<p>Some fixes store an internal “state” which is written to binary
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restart files via the <a class="reference internal" href="restart.html"><em>restart</em></a> or
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<a class="reference internal" href="write_restart.html"><em>write_restart</em></a> 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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<a class="reference internal" href="read_restart.html"><em>read_restart</em></a> 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.</p>
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<hr class="docutils" />
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<p>Some fixes calculate one of three styles of quantities: global,
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per-atom, or local, which can be used by other commands or output as
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described below. A global quantity is one or more system-wide values,
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e.g. the energy of a wall interacting with particles. A per-atom
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quantity is one or more values per atom, e.g. the displacement vector
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for each atom since time 0. Per-atom values are set to 0.0 for atoms
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not in the specified fix group. Local quantities are calculated by
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each processor based on the atoms it owns, but there may be zero or
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more per atoms.</p>
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<p>Note that a single fix may produces either global or per-atom or local
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quantities (or none at all), but never more than one of these.</p>
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<p>Global, per-atom, and local quantities each come in three kinds: a
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single scalar value, a vector of values, or a 2d array of values. The
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doc page for each fix describes the style and kind of values it
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produces, e.g. a per-atom vector. Some fixes produce more than one
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kind of a single style, e.g. a global scalar and a global vector.</p>
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<p>When a fix quantity is accessed, as in many of the output commands
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discussed below, it can be referenced via the following bracket
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notation, where ID is the ID of the fix:</p>
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<table border="1" class="docutils">
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<colgroup>
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<col width="21%" />
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<col width="79%" />
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</colgroup>
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<tbody valign="top">
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<tr class="row-odd"><td>f_ID</td>
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<td>entire scalar, vector, or array</td>
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</tr>
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<tr class="row-even"><td>f_ID[I]</td>
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<td>one element of vector, one column of array</td>
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</tr>
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<tr class="row-odd"><td>f_ID[I][J]</td>
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<td>one element of array</td>
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</tr>
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</tbody>
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</table>
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<p>In other words, using one bracket reduces the dimension of the
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quantity once (vector -> scalar, array -> vector). Using two brackets
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reduces the dimension twice (array -> scalar). Thus a command that
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uses scalar fix values as input can also process elements of a vector
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or array.</p>
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<p>Note that commands and <a class="reference internal" href="variable.html"><em>variables</em></a> which use fix
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quantities typically do not allow for all kinds, e.g. a command may
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require a vector of values, not a scalar. This means there is no
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ambiguity about referring to a fix quantity as f_ID even if it
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produces, for example, both a scalar and vector. The doc pages for
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various commands explain the details.</p>
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<hr class="docutils" />
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<p>In LAMMPS, the values generated by a fix can be used in several ways:</p>
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<ul class="simple">
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<li>Global values can be output via the <a class="reference internal" href="thermo_style.html"><em>thermo_style custom</em></a> or <a class="reference internal" href="fix_ave_time.html"><em>fix ave/time</em></a> command.
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Or the values can be referenced in a <a class="reference internal" href="variable.html"><em>variable equal</em></a> or
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<a class="reference internal" href="variable.html"><em>variable atom</em></a> command.</li>
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<li>Per-atom values can be output via the <a class="reference internal" href="dump.html"><em>dump custom</em></a> command
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or the <a class="reference internal" href="fix_ave_spatial.html"><em>fix ave/spatial</em></a> command. Or they can be
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time-averaged via the <a class="reference internal" href="fix_ave_atom.html"><em>fix ave/atom</em></a> command or
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reduced by the <a class="reference internal" href="compute_reduce.html"><em>compute reduce</em></a> command. Or the
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per-atom values can be referenced in an <a class="reference internal" href="variable.html"><em>atom-style variable</em></a>.</li>
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<li>Local values can be reduced by the <a class="reference internal" href="compute_reduce.html"><em>compute reduce</em></a> command, or histogrammed by the <a class="reference internal" href="fix_ave_histo.html"><em>fix ave/histo</em></a> command.</li>
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</ul>
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<p>See this <a class="reference internal" href="Section_howto.html#howto-15"><span>howto section</span></a> for a summary of
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various LAMMPS output options, many of which involve fixes.</p>
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<p>The results of fixes that calculate global quantities can be either
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“intensive” or “extensive” values. Intensive means the value is
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independent of the number of atoms in the simulation,
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e.g. temperature. Extensive means the value scales with the number of
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atoms in the simulation, e.g. total rotational kinetic energy.
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<a class="reference internal" href="thermo_style.html"><em>Thermodynamic output</em></a> will normalize extensive
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values by the number of atoms in the system, depending on the
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“thermo_modify norm” setting. It will not normalize intensive values.
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If a fix value is accessed in another way, e.g. by a
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<a class="reference internal" href="variable.html"><em>variable</em></a>, you may want to know whether it is an
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intensive or extensive value. See the doc page for individual fixes
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for further info.</p>
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<hr class="docutils" />
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<p>Each fix style has its own documentation page which describes its
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arguments and what it does, as listed below. Here is an alphabetic
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list of fix styles available in LAMMPS. They are also given in more
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compact form in the Fix section of <a class="reference internal" href="Section_commands.html#cmd-5"><span>this page</span></a>.</p>
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<p>There are also additional fix styles (not listed here) submitted by
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users which are included in the LAMMPS distribution. The list of
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these with links to the individual styles are given in the fix section
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of <a class="reference internal" href="Section_commands.html#cmd-5"><span>this page</span></a>.</p>
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<ul class="simple">
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<li><a class="reference internal" href="fix_adapt.html"><em>adapt</em></a> - change a simulation parameter over time</li>
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<li><a class="reference internal" href="fix_addforce.html"><em>addforce</em></a> - add a force to each atom</li>
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<li><a class="reference internal" href="fix_append_atoms.html"><em>append/atoms</em></a> - append atoms to a running simulation</li>
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<li><a class="reference internal" href="fix_atom_swap.html"><em>atom/swap</em></a> - Monte Carlo atom type swapping</li>
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<li><a class="reference internal" href="fix_aveforce.html"><em>aveforce</em></a> - add an averaged force to each atom</li>
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<li><a class="reference internal" href="fix_ave_atom.html"><em>ave/atom</em></a> - compute per-atom time-averaged quantities</li>
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<li><a class="reference internal" href="fix_ave_chunk.html"><em>ave/chunk</em></a> - compute per-chunk time-averaged quantities</li>
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<li><a class="reference internal" href="fix_ave_correlate.html"><em>ave/correlate</em></a> - compute/output time correlations</li>
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<li><a class="reference internal" href="fix_ave_histo.html"><em>ave/histo</em></a> - compute/output time-averaged histograms</li>
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<li><a class="reference internal" href="fix_ave_spatial.html"><em>ave/spatial</em></a> - compute/output time-averaged per-atom quantities by layer</li>
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<li><a class="reference internal" href="fix_ave_time.html"><em>ave/time</em></a> - compute/output global time-averaged quantities</li>
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<li><a class="reference internal" href="fix_balance.html"><em>balance</em></a> - perform dynamic load-balancing</li>
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<li><a class="reference internal" href="fix_bond_break.html"><em>bond/break</em></a> - break bonds on the fly</li>
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<li><a class="reference internal" href="fix_bond_create.html"><em>bond/create</em></a> - create bonds on the fly</li>
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<li><a class="reference internal" href="fix_bond_swap.html"><em>bond/swap</em></a> - Monte Carlo bond swapping</li>
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<li><a class="reference internal" href="fix_box_relax.html"><em>box/relax</em></a> - relax box size during energy minimization</li>
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<li><a class="reference internal" href="fix_deform.html"><em>deform</em></a> - change the simulation box size/shape</li>
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<li><a class="reference internal" href="fix_deposit.html"><em>deposit</em></a> - add new atoms above a surface</li>
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<li><a class="reference internal" href="fix_drag.html"><em>drag</em></a> - drag atoms towards a defined coordinate</li>
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<li><a class="reference internal" href="fix_dt_reset.html"><em>dt/reset</em></a> - reset the timestep based on velocity, forces</li>
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<li><a class="reference internal" href="fix_efield.html"><em>efield</em></a> - impose electric field on system</li>
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<li><a class="reference internal" href="fix_enforce2d.html"><em>enforce2d</em></a> - zero out z-dimension velocity and force</li>
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<li><a class="reference internal" href="fix_evaporate.html"><em>evaporate</em></a> - remove atoms from simulation periodically</li>
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<li><a class="reference internal" href="fix_external.html"><em>external</em></a> - callback to an external driver program</li>
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<li><a class="reference internal" href="fix_freeze.html"><em>freeze</em></a> - freeze atoms in a granular simulation</li>
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<li><a class="reference internal" href="fix_gcmc.html"><em>gcmc</em></a> - grand canonical insertions/deletions</li>
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<li><a class="reference internal" href="fix_gcmc.html"><em>gld</em></a> - generalized Langevin dynamics integrator</li>
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<li><a class="reference internal" href="fix_gravity.html"><em>gravity</em></a> - add gravity to atoms in a granular simulation</li>
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<li><a class="reference internal" href="fix_heat.html"><em>heat</em></a> - add/subtract momentum-conserving heat</li>
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<li><a class="reference internal" href="fix_indent.html"><em>indent</em></a> - impose force due to an indenter</li>
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<li><a class="reference internal" href="fix_langevin.html"><em>langevin</em></a> - Langevin temperature control</li>
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<li><a class="reference internal" href="fix_lineforce.html"><em>lineforce</em></a> - constrain atoms to move in a line</li>
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<li><a class="reference internal" href="fix_momentum.html"><em>momentum</em></a> - zero the linear and/or angular momentum of a group of atoms</li>
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<li><a class="reference internal" href="fix_move.html"><em>move</em></a> - move atoms in a prescribed fashion</li>
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<li><a class="reference internal" href="fix_msst.html"><em>msst</em></a> - multi-scale shock technique (MSST) integration</li>
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<li><a class="reference internal" href="fix_neb.html"><em>neb</em></a> - nudged elastic band (NEB) spring forces</li>
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<li><a class="reference internal" href="fix_nh.html"><em>nph</em></a> - constant NPH time integration via Nose/Hoover</li>
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<li><a class="reference internal" href="fix_nphug.html"><em>nphug</em></a> - constant-stress Hugoniostat integration</li>
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<li><a class="reference internal" href="fix_nph_asphere.html"><em>nph/asphere</em></a> - NPH for aspherical particles</li>
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<li><a class="reference internal" href="fix_nve_body.html"><em>nph/body</em></a> - NPH for body particles</li>
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<li><a class="reference internal" href="fix_nph_sphere.html"><em>nph/sphere</em></a> - NPH for spherical particles</li>
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<li><a class="reference internal" href="fix_nh.html"><em>npt</em></a> - constant NPT time integration via Nose/Hoover</li>
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<li><a class="reference internal" href="fix_npt_asphere.html"><em>npt/asphere</em></a> - NPT for aspherical particles</li>
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<li><a class="reference internal" href="fix_nve_body.html"><em>npt/body</em></a> - NPT for body particles</li>
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<li><a class="reference internal" href="fix_npt_sphere.html"><em>npt/sphere</em></a> - NPT for spherical particles</li>
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<li><a class="reference internal" href="fix_nve.html"><em>nve</em></a> - constant NVE time integration</li>
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<li><a class="reference internal" href="fix_nve_asphere.html"><em>nve/asphere</em></a> - NVE for aspherical particles</li>
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<li><a class="reference internal" href="fix_nve_asphere_noforce.html"><em>nve/asphere/noforce</em></a> - NVE for aspherical particles without forces”</li>
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<li><a class="reference internal" href="fix_nve_body.html"><em>nve/body</em></a> - NVE for body particles</li>
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<li><a class="reference internal" href="fix_nve_limit.html"><em>nve/limit</em></a> - NVE with limited step length</li>
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<li><a class="reference internal" href="fix_nve_line.html"><em>nve/line</em></a> - NVE for line segments</li>
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<li><a class="reference internal" href="fix_nve_noforce.html"><em>nve/noforce</em></a> - NVE without forces (v only)</li>
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<li><a class="reference internal" href="fix_nve_sphere.html"><em>nve/sphere</em></a> - NVE for spherical particles</li>
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<li><a class="reference internal" href="fix_nve_tri.html"><em>nve/tri</em></a> - NVE for triangles</li>
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<li><a class="reference internal" href="fix_nh.html"><em>nvt</em></a> - constant NVT time integration via Nose/Hoover</li>
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<li><a class="reference internal" href="fix_nvt_asphere.html"><em>nvt/asphere</em></a> - NVT for aspherical particles</li>
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<li><a class="reference internal" href="fix_nve_body.html"><em>nvt/body</em></a> - NVT for body particles</li>
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<li><a class="reference internal" href="fix_nvt_sllod.html"><em>nvt/sllod</em></a> - NVT for NEMD with SLLOD equations</li>
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<li><a class="reference internal" href="fix_nvt_sphere.html"><em>nvt/sphere</em></a> - NVT for spherical particles</li>
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<li><a class="reference internal" href="fix_oneway.html"><em>oneway</em></a> - constrain particles on move in one direction</li>
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<li><a class="reference internal" href="fix_orient_fcc.html"><em>orient/fcc</em></a> - add grain boundary migration force</li>
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<li><a class="reference internal" href="fix_planeforce.html"><em>planeforce</em></a> - constrain atoms to move in a plane</li>
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<li><a class="reference internal" href="fix_poems.html"><em>poems</em></a> - constrain clusters of atoms to move as coupled rigid bodies</li>
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<li><a class="reference internal" href="fix_pour.html"><em>pour</em></a> - pour new atoms/molecules into a granular simulation domain</li>
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<li><a class="reference internal" href="fix_press_berendsen.html"><em>press/berendsen</em></a> - pressure control by Berendsen barostat</li>
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<li><a class="reference internal" href="fix_print.html"><em>print</em></a> - print text and variables during a simulation</li>
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<li><a class="reference internal" href="fix_property_atom.html"><em>property/atom</em></a> - add customized per-atom values</li>
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<li><a class="reference internal" href="fix_qeq_comb.html"><em>qeq/comb</em></a> - charge equilibration for COMB potential <a class="reference internal" href="fix_qeq.html"><em>qeq/dynamic</em></a> - charge equilibration via dynamic method <a class="reference internal" href="fix_qeq.html"><em>qeq/fire</em></a> - charge equilibration via FIRE minimizer <a class="reference internal" href="fix_qeq.html"><em>qeq/point</em></a> - charge equilibration via point method <a class="reference internal" href="fix_qeq.html"><em>qeq/shielded</em></a> - charge equilibration via shielded method <a class="reference internal" href="fix_qeq.html"><em>qeq/slater</em></a> - charge equilibration via Slater method <a class="reference internal" href="fix_shake.html"><em>rattle</em></a> - RATTLE constraints on bonds and/or angles</li>
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<li><a class="reference internal" href="fix_reax_bonds.html"><em>reax/bonds</em></a> - write out ReaxFF bond information <a class="reference internal" href="fix_recenter.html"><em>recenter</em></a> - constrain the center-of-mass position of a group of atoms</li>
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<li><a class="reference internal" href="fix_restrain.html"><em>restrain</em></a> - constrain a bond, angle, dihedral</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid</em></a> - constrain one or more clusters of atoms to move as a rigid body with NVE integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/nph</em></a> - constrain one or more clusters of atoms to move as a rigid body with NPH integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/npt</em></a> - constrain one or more clusters of atoms to move as a rigid body with NPT integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/nve</em></a> - constrain one or more clusters of atoms to move as a rigid body with alternate NVE integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/nvt</em></a> - constrain one or more clusters of atoms to move as a rigid body with NVT integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/small</em></a> - constrain many small clusters of atoms to move as a rigid body with NVE integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/small/nph</em></a> - constrain many small clusters of atoms to move as a rigid body with NPH integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/small/npt</em></a> - constrain many small clusters of atoms to move as a rigid body with NPT integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/small/nve</em></a> - constrain many small clusters of atoms to move as a rigid body with alternate NVE integration</li>
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<li><a class="reference internal" href="fix_rigid.html"><em>rigid/small/nvt</em></a> - constrain many small clusters of atoms to move as a rigid body with NVT integration</li>
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<li><a class="reference internal" href="fix_setforce.html"><em>setforce</em></a> - set the force on each atom</li>
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<li><a class="reference internal" href="fix_shake.html"><em>shake</em></a> - SHAKE constraints on bonds and/or angles</li>
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<li><a class="reference internal" href="fix_spring.html"><em>spring</em></a> - apply harmonic spring force to group of atoms</li>
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<li><a class="reference internal" href="fix_spring_rg.html"><em>spring/rg</em></a> - spring on radius of gyration of group of atoms</li>
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<li><a class="reference internal" href="fix_spring_self.html"><em>spring/self</em></a> - spring from each atom to its origin</li>
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<li><a class="reference internal" href="fix_srd.html"><em>srd</em></a> - stochastic rotation dynamics (SRD)</li>
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<li><a class="reference internal" href="fix_store_force.html"><em>store/force</em></a> - store force on each atom</li>
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<li><a class="reference internal" href="fix_store_state.html"><em>store/state</em></a> - store attributes for each atom</li>
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<li><a class="reference internal" href="fix_temp_berendsen.html"><em>temp/berendsen</em></a> - temperature control by Berendsen thermostat</li>
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<li><a class="reference internal" href="fix_temp_csvr.html"><em>temp/csld</em></a> - canonical sampling thermostat with Langevin dynamics</li>
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<li><a class="reference internal" href="fix_temp_csvr.html"><em>temp/csvr</em></a> - canonical sampling thermostat with Hamiltonian dynamics</li>
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<li><a class="reference internal" href="fix_temp_rescale.html"><em>temp/rescale</em></a> - temperature control by velocity rescaling</li>
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<li><a class="reference internal" href="fix_tfmc.html"><em>tfmc</em></a> - perform force-bias Monte Carlo with time-stamped method</li>
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<li><a class="reference internal" href="fix_thermal_conductivity.html"><em>thermal/conductivity</em></a> - Muller-Plathe kinetic energy exchange for thermal conductivity calculation</li>
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<li><a class="reference internal" href="fix_tmd.html"><em>tmd</em></a> - guide a group of atoms to a new configuration</li>
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<li><a class="reference internal" href="fix_ttm.html"><em>ttm</em></a> - two-temperature model for electronic/atomic coupling</li>
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<li><a class="reference internal" href="fix_tune_kspace.html"><em>tune/kspace</em></a> - auto-tune KSpace parameters</li>
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<li><a class="reference internal" href="fix_vector.html"><em>vector</em></a> - accumulate a global vector every N timesteps</li>
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<li><a class="reference internal" href="fix_viscosity.html"><em>viscosity</em></a> - Muller-Plathe momentum exchange for viscosity calculation</li>
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<li><a class="reference internal" href="fix_viscous.html"><em>viscous</em></a> - viscous damping for granular simulations</li>
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<li><a class="reference internal" href="fix_wall.html"><em>wall/colloid</em></a> - Lennard-Jones wall interacting with finite-size particles</li>
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<li><a class="reference internal" href="fix_wall_gran.html"><em>wall/gran</em></a> - frictional wall(s) for granular simulations</li>
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<li><a class="reference internal" href="fix_wall.html"><em>wall/harmonic</em></a> - harmonic spring wall</li>
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<li><a class="reference internal" href="fix_wall.html"><em>wall/lj1043</em></a> - Lennard-Jones 10-4-3 wall</li>
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<li><a class="reference internal" href="fix_wall.html"><em>wall/lj126</em></a> - Lennard-Jones 12-6 wall</li>
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<li><a class="reference internal" href="fix_wall.html"><em>wall/lj93</em></a> - Lennard-Jones 9-3 wall</li>
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<li><a class="reference internal" href="fix_wall_piston.html"><em>wall/piston</em></a> - moving reflective piston wall</li>
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<li><a class="reference internal" href="fix_wall_reflect.html"><em>wall/reflect</em></a> - reflecting wall(s)</li>
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<li><a class="reference internal" href="fix_wall_region.html"><em>wall/region</em></a> - use region surface as wall</li>
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<li><a class="reference internal" href="fix_wall_srd.html"><em>wall/srd</em></a> - slip/no-slip wall for SRD particles</li>
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</ul>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>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 <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info on packages.
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The doc pages for individual fixes tell if it is part of a package.</p>
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</div>
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<div class="section" id="related-commands">
|
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
|
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<p><a class="reference internal" href="unfix.html"><em>unfix</em></a>, <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a></p>
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<p><strong>Default:</strong> none</p>
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</div>
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