# restart command

## Syntax

restart 0
restart N root keyword value ...
restart N file1 file2 keyword value ...

• N = write a restart file every this many timesteps

• N can be a variable (see below)

• root = filename to which timestep # is appended

• file1,file2 = two full filenames, toggle between them when writing file

• zero or more keyword/value pairs may be appended

• keyword = fileper or nfile

fileper arg = Np
Np = write one file for every this many processors
nfile arg = Nf
Nf = write this many files, one from each of Nf processors


## Examples

restart 0
restart 1000 poly.restart
restart 1000 poly.restart.mpiio
restart 1000 restart.*.equil
restart 10000 poly.%.1 poly.%.2 nfile 10
restart v_mystep poly.restart


## Description

Write out a binary restart file with the current state of the simulation every so many timesteps, in either or both of two modes, as a run proceeds. A value of 0 means do not write out any restart files. The two modes are as follows. If one filename is specified, a series of filenames will be created which include the timestep in the filename. If two filenames are specified, only 2 restart files will be created, with those names. LAMMPS will toggle between the 2 names as it writes successive restart files.

Note that you can specify the restart command twice, once with a single filename and once with two filenames. This would allow you, for example, to write out archival restart files every 100000 steps using a single filenname, and more frequent temporary restart files every 1000 steps, using two filenames. Using restart 0 will turn off both modes of output.

Similar to dump files, the restart filename(s) can contain two wild-card characters.

If a “*” appears in the single filename, it is replaced with the current timestep value. This is only recognized when a single filename is used (not when toggling back and forth). Thus, the 3rd example above creates restart files as follows: restart.1000.equil, restart.2000.equil, etc. If a single filename is used with no “*”, then the timestep value is appended. E.g. the 2nd example above creates restart files as follows: poly.restart.1000, poly.restart.2000, etc.

If a “%” character appears in the restart filename(s), then one file is written for each processor and the “%” character is replaced with the processor ID from 0 to P-1. An additional file with the “%” replaced by “base” is also written, which contains global information. For example, the files written on step 1000 for filename restart.% would be restart.base.1000, restart.0.1000, restart.1.1000, …, restart.P-1.1000. This creates smaller files and can be a fast mode of output and subsequent input on parallel machines that support parallel I/O. The optional fileper and nfile keywords discussed below can alter the number of files written.

The restart file can also be written in parallel as one large binary file via the MPI-IO library, which 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.

make yes-mpiio    # installs the MPIIO package
make mpi          # build LAMMPS for your platform


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.

Restart files are written on timesteps that are a multiple of N but not on the first timestep of a run or minimization. You can use the write_restart command to write a restart file before a run begins. A restart file is not written on the last timestep of a run unless it is a multiple of N. A restart file is written on the last timestep of a minimization if N > 0 and the minimization converges.

Instead of a numeric value, N can be specified as an equal-style variable, which should be specified as v_name, where name is the variable name. In this case, the variable is evaluated at the beginning of a run to determine the next timestep at which a restart file will be written out. On that timestep, the variable will be evaluated again to determine the next timestep, etc. Thus the variable should return timestep values. See the stagger() and logfreq() and stride() math functions for equal-style variables, as examples of useful functions to use in this context. Other similar math functions could easily be added as options for equal-style variables.

For example, the following commands will write restart files every step from 1100 to 1200, and could be useful for debugging a simulation where something goes wrong at step 1163:

variable       s equal stride(1100,1200,1)
restart        v_s tmp.restart


See the read_restart command for information about what is stored in a restart file.

Restart files can be read by a read_restart command to restart a simulation from a particular state. Because the file is binary (to enable exact restarts), it may not be readable on another machine. In this case, you can use the -r command-line switch to convert a restart file to a data file.

Note

Although the purpose of restart files is to enable restarting a simulation from where it left off, not all information about a simulation is stored in the file. For example, the list of fixes that were specified during the initial run is not stored, which means the new input script must specify any fixes you want to use. Even when restart information is stored in the file, as it is for some fixes, commands may need to be re-specified in the new input script, in order to re-use that information. See the read_restart command for information about what is stored in a restart file.

The optional nfile or fileper keywords can be used in conjunction with the “%” wildcard character in the specified restart file name(s). As explained above, the “%” character causes the restart file to be written in pieces, one piece for each of P processors. By default P = the number of processors the simulation is running on. The nfile or fileper keyword can be used to set P to a smaller value, which can be more efficient when running on a large number of processors.

The nfile keyword sets P to the specified Nf value. For example, if Nf = 4, and the simulation is running on 100 processors, 4 files will be written, by processors 0,25,50,75. Each will collect information from itself and the next 24 processors and write it to a restart file.

For the fileper keyword, the specified value of Np means write one file for every Np processors. For example, if Np = 4, every 4th processor (0,4,8,12,etc) will collect information from itself and the next 3 processors and write it to a restart file.

## Restrictions

To write and read restart files in parallel with MPI-IO, the MPIIO package must be installed.

## Default

restart 0