forked from lijiext/lammps
176 lines
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HTML
176 lines
8.2 KiB
HTML
<HTML>
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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>
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<HR>
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<H3>read_restart command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>read_restart file
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</PRE>
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<UL><LI>file = name of binary restart file to read in
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>read_restart save.10000
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read_restart restart.*
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read_restart poly.*.%
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</PRE>
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<PRE>
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Read in a previously saved simulation from a restart file. This
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allows continuation of a previous run. Information about what is
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stored in a restart file is given below.
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</P>
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<P>Restart files are saved in binary format to enable exact restarts,
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meaning that the trajectories of a restarted run will precisely match
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those produced by the original run had it continued on.
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</P>
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<P>Several things can prevent exact restarts due to round-off effects, in
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which case the trajectories in the 2 runs will slowly diverge. These
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include running on a different number of processors or changing
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certain settings such as those set by the <A HREF = "newton.html">newton</A> or
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<A HREF = "processors.html">processors</A> commands. LAMMPS will issue a warning in
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these cases.
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</P>
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<P>Certain fixes will not restart exactly, though they should provide
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statistically similar results. These include <A HREF = "fix_shake.html">fix
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shake</A> and <A HREF = "fix_langevin.html">fix langevin</A>.
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</P>
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<P>Certain pair styles will not restart exactly, though they should
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provide statistically similar results. This is because the forces
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they compute depend on atom velocities, which are used at half-step
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values every timestep when forces are computed. When a run restarts,
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forces are initially evaluated with a full-step velocity, which is
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different than if the run had continued. These pair styles include
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<A HREF = "pair_gran.html">granular pair styles</A>, <A HREF = "pair_dpd.html">pair dpd</A>, and
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<A HREF = "pair_lubricate.html">pair lubricate</A>.
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</P>
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<P>If a restarted run is immediately different than the run which
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produced the restart file, it could be a LAMMPS bug, so consider
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<A HREF = "Section_errors.html#err_2">reporting it</A> if you think the behavior is
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wrong.
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</P>
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<P>Because restart files are binary, they may not be portable to other
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machines. In this case, you can use the <A HREF = "Section_start.html#start_7">-r command-line
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switch</A> to convert a restart file to a data
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file.
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</P>
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<P>Similar to how restart files are written (see the
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<A HREF = "write_restart.html">write_restart</A> and <A HREF = "restart.html">restart</A>
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commands), the restart filename can contain two wild-card characters.
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If a "*" appears in the filename, the directory is searched for all
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filenames that match the pattern where "*" is replaced with a timestep
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value. The file with the largest timestep value is read in. Thus,
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this effectively means, read the latest restart file. It's useful if
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you want your script to continue a run from where it left off. See
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the <A HREF = "run.html">run</A> command and its "upto" option for how to specify
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the run command so it doesn't need to be changed either.
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</P>
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<P>If a "%" character appears in the restart filename, LAMMPS expects a
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set of multiple files to exist. The <A HREF = "restart.html">restart</A> and
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<A HREF = "write_restart.html">write_restart</A> commands explain how such sets are
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created. Read_restart will first read a filename where "%" is
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replaced by "base". This file tells LAMMPS how many processors
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created the set and how many files are in it. Read_restart then reads
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the additional files. For example, if the restart file was specified
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as save.% when it was written, then read_restart reads the files
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save.base, save.0, save.1, ... save.P-1, where P is the number of
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processors that created the restart file.
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</P>
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<P>Note that P could be the total number of processors in the previous
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simulation, or some subset of those processors, if the <I>fileper</I> or
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<I>nfile</I> options were used when the restart file was written; see the
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<A HREF = "restart.html">restart</A> and <A HREF = "write_restart.html">write_restart</A> commands
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for details. The processors in the current LAMMPS simulation share
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the work of reading these files; each reads a roughly equal subset of
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the files. The number of processors which created the set can be
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different the number of processors in the current LAMMPS simulation.
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This can be a fast mode of input on parallel machines that support
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parallel I/O.
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</P>
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<HR>
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<P>A restart file stores the following information about a simulation:
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units and atom style, simulation box size and shape and boundary
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settings, group definitions, per-type atom settings such as mass,
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per-atom attributes including their group assignments and molecular
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topology attributes, force field styles and coefficients, and
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<A HREF = "special_bonds.html">special_bonds</A> settings. This means that commands
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for these quantities do not need to be re-specified in the input
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script that reads the restart file, though you can redefine settings
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after the restart file is read.
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</P>
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<P>One exception is that some pair styles do not store their info in
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restart files. Typically these are many-body potentials which read
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their parameters from separate files; you need to re-specify the <A HREF = "pair_style.html">pair
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style</A> and <A HREF = "pair_coeff.html">pair coeff</A> commands in
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your restart input script. The doc pages for individual pair styles
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note if this is the case. This is also true of bond_style hybrid (and
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angle_style, dihedral_style, improper_style hybrid).
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</P>
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<P>All settings made by the <A HREF = "pair_modify.html">pair_modify</A> command, such
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as the shift and tail settings, are stored in the restart file with
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the pair style. The one exception is the <A HREF = "pair_modify.html">pair_modify
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compute</A> setting is not stored, since it is typically
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only used for debugging purposes.
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</P>
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<P>Information about <A HREF = "kspace_style.html">kspace_style</A> settings are not
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stored in the restart file. Hence if you wish to use an Ewald or PPPM
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solver, these commands must be re-issued after the restart file is
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read.
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</P>
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<P>The list of <A HREF = "fix.html">fixes</A> used for a simulation is not stored in
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the restart file. This means the new input script should specify all
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fixes it will use. Note that some fixes store an internal "state"
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which is written to the restart file. This allows the fix to continue
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on with its calculations in a restarted simulation. To re-enable such
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a fix, the fix command in the new input script must use the same
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fix-ID and group-ID as was used in the input script that wrote the
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restart file. If a match is found, LAMMPS prints a message indicating
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that the fix is being re-enabled. If no match is found before the
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first run or minimization is performed by the new script, the "state"
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information for the saved fix is discarded. See the doc pages for
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individual fixes for info on which ones can be restarted in this
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manner.
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</P>
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<P>Bond interactions (angle, etc) that have been turned off by the <A HREF = "fix_shake.html">fix
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shake</A> or <A HREF = "delete_bonds.html">delete_bonds</A> command will
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be written to a restart file as if they are turned on. This means
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they will need to be turned off again in a new run after the restart
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file is read.
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</P>
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<P>Bonds that are broken (e.g. by a bond-breaking potential) are written
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to the restart file as broken bonds with a type of 0. Thus these
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bonds will still be broken when the restart file is read.
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</P>
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<P>IMPORTANT NOTE: No other information is stored in the restart file.
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This means that an input script that reads a restart file should
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specify settings for quantities like <A HREF = "timestep.html">timestep size</A>,
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<A HREF = "thermo_style.html">thermodynamic</A>, <A HREF = "neighbor.html">neighbor list</A>
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criteria including settings made via the
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<A HREF = "neigh_modify.html">neigh_modify</A> comand, <A HREF = "dump.html">dump</A> file
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output, <A HREF = "region.html">geometric regions</A>, etc.
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</P>
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<HR>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "read_data.html">read_data</A>, <A HREF = "read_dump.html">read_dump</A>,
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<A HREF = "write_restart.html">write_restart</A>, <A HREF = "restart.html">restart</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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