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### write_data command

Syntax:

    write_data file

- file = name of data file to write out

Examples:

    write_data data.polymer
    write_data data.*

Description:

Write a data file in text format of the current state of the simulation. Data files can be read by the read data command to begin a simulation. The read_data command also describes their format.

Similar to dump files, the data filename can contain a "*" wild-card character. The "*" is replaced with the current timestep value.

IMPORTANT NOTE: The write-data command is not yet fully implemented in two respects. First, most pair, bond, angle, dihedral, improper styles do not yet write their coefficient information into the data file. This means you will need to specify that information in your input script that reads the data file, e.g. with the pair_coeff command. Second, a few of the atom styles (body, ellipsoid, line, tri) that store auxiliary "bonus" information about aspherical particles, do not yet write the bonus info into the data file. Both these functionalities will be added to the write_data command later.

Because a data file is in text format, if you use a data file written out by this command to restart a simulation, the initial state of the new run will be slightly different than the final state of the old run (when the file was written) which was represented internally by LAMMPS in binary format. A new simulation which reads the data file will thus typically diverge from a simulation that continued in the original input script.

If you want to do more exact restarts, using binary files, see the restart, write_restart, and read_restart commands. You can also convert binary restart files to text data files, after a simulation has run, using the restart2data tool in the tools directory. When the write_data command is fully implemented (see NOTE above), the restart2data command will be obsolete and will be removed from the LAMMPS distribution.

IMPORTANT NOTE: Only limited information about a simulation is stored in a data file. For example, no information about atom groups and fixes are stored. Binary restart files store more information.

Bond interactions (angle, etc) that have been turned off by the fix shake or delete_bonds command will be written to a data file as if they are turned on. This means they will need to be turned off again in a new run after the data file is read.

Bonds that are broken (e.g. by a bond-breaking potential) are not written to the data file. Thus these bonds will not exist when the data file is read.

Restrictions:

This command requires inter-processor communication to migrate atoms before the data file is written. This means that your system must be ready to perform a simulation before using this command (force fields setup, atom masses initialized, etc).

Related commands:

read_data, write_restart

Default: none