<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
<p>Read in a previously saved system configuration from a restart file.
This allows continuation of a previous run. Details about what
information is stored (and not stored) in a restart file is given
below. Basically this operation will re-create the simulation box
with all its atoms and their attributes as well as some related global
settings, at the point in time it was written to the restart file by a
previous simluation. The simulation box will be partitioned into a
regular 3d grid of rectangular bricks, one per processor, based on the
number of processors in the current simulation and the settings of the
<a class="reference internal" href="processors.html"><em>processors</em></a> command. The partitioning can later be
changed by the <a class="reference internal" href="balance.html"><em>balance</em></a> or <a class="reference internal" href="fix_balance.html"><em>fix balance</em></a> commands.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Normally, restart files are written by the
<a class="reference internal" href="restart.html"><em>restart</em></a> or <a class="reference internal" href="write_restart.html"><em>write_restart</em></a> commands
so that all atoms in the restart file are inside the simulation box.
If this is not the case, the read_restart command will print an error
that atoms were “lost” when the file is read. This error should be
reported to the LAMMPS developers so the invalid writing of the
restart file can be fixed. If you still wish to use the restart file,
the optional <em>remap</em> flag can be appended to the read_restart command.
This should avoid the error, by explicitly remapping each atom back into
the simulation box, updating image flags for the atom appropriately.</p>
</div>
<p>Restart files are saved in binary format to enable exact restarts,
meaning that the trajectories of a restarted run will precisely match
those produced by the original run had it continued on.</p>
<p>Several things can prevent exact restarts due to round-off effects, in
which case the trajectories in the 2 runs will slowly diverge. These
include running on a different number of processors or changing
certain settings such as those set by the <a class="reference internal" href="newton.html"><em>newton</em></a> or
<a class="reference internal" href="processors.html"><em>processors</em></a> commands. LAMMPS will issue a warning in
these cases.</p>
<p>Certain fixes will not restart exactly, though they should provide
statistically similar results. These include <a class="reference internal" href="fix_shake.html"><em>fix shake</em></a> and <a class="reference internal" href="fix_langevin.html"><em>fix langevin</em></a>.</p>
<p>Certain pair styles will not restart exactly, though they should
provide statistically similar results. This is because the forces
they compute depend on atom velocities, which are used at half-step
values every timestep when forces are computed. When a run restarts,
forces are initially evaluated with a full-step velocity, which is
different than if the run had continued. These pair styles include
<p>If a restarted run is immediately different than the run which
produced the restart file, it could be a LAMMPS bug, so consider
<a class="reference internal" href="Section_errors.html#err-2"><span>reporting it</span></a> if you think the behavior is
wrong.</p>
<p>Because restart files are binary, they may not be portable to other
machines. In this case, you can use the <a class="reference internal" href="Section_start.html#start-7"><span>-restart command-line switch</span></a> to convert a restart file to a data
file.</p>
<p>Similar to how restart files are written (see the
<a class="reference internal" href="write_restart.html"><em>write_restart</em></a> and <a class="reference internal" href="restart.html"><em>restart</em></a>
commands), the restart filename can contain two wild-card characters.
If a “*” appears in the filename, the directory is searched for all
filenames that match the pattern where “*” is replaced with a timestep
value. The file with the largest timestep value is read in. Thus,
this effectively means, read the latest restart file. It’s useful if
you want your script to continue a run from where it left off. See
the <a class="reference internal" href="run.html"><em>run</em></a> command and its “upto” option for how to specify
the run command so it doesn’t need to be changed either.</p>
<p>If a “%” character appears in the restart filename, LAMMPS expects a
set of multiple files to exist. The <a class="reference internal" href="restart.html"><em>restart</em></a> and
<a class="reference internal" href="write_restart.html"><em>write_restart</em></a> commands explain how such sets are
created. Read_restart will first read a filename where “%” is
replaced by “base”. This file tells LAMMPS how many processors
created the set and how many files are in it. Read_restart then reads
the additional files. For example, if the restart file was specified
as save.% when it was written, then read_restart reads the files
save.base, save.0, save.1, ... save.P-1, where P is the number of
processors that created the restart file.</p>
<p>Note that P could be the total number of processors in the previous
simulation, or some subset of those processors, if the <em>fileper</em> or
<em>nfile</em> options were used when the restart file was written; see the
<a class="reference internal" href="restart.html"><em>restart</em></a> and <a class="reference internal" href="write_restart.html"><em>write_restart</em></a> commands
for details. The processors in the current LAMMPS simulation share
the work of reading these files; each reads a roughly equal subset of
the files. The number of processors which created the set can be
different the number of processors in the current LAMMPS simulation.
This can be a fast mode of input on parallel machines that support
parallel I/O.</p>
<p>A restart file can also be read in parallel as one large binary file
via the MPI-IO library, assuming it was also written with MPI-IO.
MPI-IO is part of the MPI standard for versions 2.0 and above. Using
MPI-IO requires two steps. First, build LAMMPS with its MPIIO package
installed, e.g.</p>
<div class="highlight-python"><div class="highlight"><pre>make yes-mpiio # installs the MPIIO package
make g++ # build LAMMPS for your platform
</pre></div>
</div>
<p>Second, use a restart filename which contains ”.mpiio”. Note that it
does not have to end in ”.mpiio”, just contain those characters.
Unlike MPI-IO dump files, a particular restart file must be both
written and read using MPI-IO.</p>
<hr class="docutils" />
<p>A restart file stores the following information about a simulation:
units, atom style and atom style related settings (<em>id</em>, <em>map</em>,
<em>sort</em>), communication settings (<em>mode</em>, <em>cutoff</em>, <em>vel</em>), timestep,
simulation box size and shape and boundary settings, group
definitions, per-type atom settings such as mass, per-atom attributes
including their group assignments and molecular topology attributes,
force field styles and - in most cases - force field coefficients, and
<a class="reference internal" href="special_bonds.html"><em>special_bonds</em></a> settings. This means that commands
for these quantities do not need to be re-specified in the input
script that reads the restart file, though you can redefine settings
after the restart file is read.</p>
<p>One exception is that some force styles (pair, bond, angle, etc) do
not store their coefficient info in restart files. Typically these
are many-body or tabulated potentials which read their parameters from
separate files; you may need to re-specify the <a class="reference internal" href="pair_style.html"><em>pair style</em></a> and <a class="reference internal" href="pair_coeff.html"><em>pair coeff</em></a> commands in
your restart input script. The doc pages for individual pair styles
mention if this is the case. This is also true of bond_style hybrid
<p>All settings made by the <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> command, such
as the shift and tail settings, are stored in the restart file with
the pair style. The one exception is the <a class="reference internal" href="pair_modify.html"><em>pair_modify compute</em></a> setting is not stored, since it is typically
only used for debugging purposes.</p>
<p>Information about <a class="reference internal" href="kspace_style.html"><em>kspace_style</em></a> settings are not
stored in the restart file. Hence if you wish to use an Ewald or PPPM
solver, these commands must be re-issued after the restart file is
read.</p>
<p>The list of <a class="reference internal" href="fix.html"><em>fixes</em></a> used for a simulation is not stored in
the restart file. This means the new input script should specify all
fixes it will use. Note that some fixes store an internal “state”
which is written to the restart file. This allows the fix to continue
on with its calculations in a restarted simulation. To re-enable such
a fix, the fix command in the new input script must use the same
fix-ID and group-ID as was used in the input script that wrote the
restart file. If a match is found, LAMMPS prints a message indicating
that the fix is being re-enabled. If no match is found before the
first run or minimization is performed by the new script, the “state”
information for the saved fix is discarded. See the doc pages for
individual fixes for info on which ones can be restarted in this
manner.</p>
<p>Bond interactions (angle, etc) that have been turned off by the <a class="reference internal" href="fix_shake.html"><em>fix shake</em></a> or <a class="reference internal" href="delete_bonds.html"><em>delete_bonds</em></a> command will
be written to a restart file as if they are turned on. This means
they will need to be turned off again in a new run after the restart
file is read.</p>
<p>Bonds that are broken (e.g. by a bond-breaking potential) are written
to the restart file as broken bonds with a type of 0. Thus these
bonds will still be broken when the restart file is read.</p>
<p>Bonds that have been broken by the <a class="reference internal" href="fix_bond_break.html"><em>fix bond/break</em></a> command have disappeared from the
system. No information about these bonds is written to the restart
file.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">No other information is stored in the restart file.
This means that an input script that reads a restart file should
specify settings for quantities like <a class="reference internal" href="timestep.html"><em>timestep size</em></a>,
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