git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@14950 f3b2605a-c512-4ea7-a41b-209d697bcdaa

doc/html/_sources/write_data.txt

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+.. index:: write_data
+
+write_data command
+==================
+
+Syntax
+""""""
+
+.. parsed-literal::
+
+   write_data file keyword value ...
+
+* file = name of data file to write out
+* zero or more keyword/value pairs may be appended
+* keyword = *pair* or *nocoeff*
+.. parsed-literal::
+
+     *nocoeff* = do not write out force field info
+     *pair* value = *ii* or *ij*
+       *ii* = write one line of pair coefficient info per atom type
+       *ij* = write one line of pair coefficient info per IJ atom type pair
+
+
+
+Examples
+""""""""
+
+.. parsed-literal::
+
+   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 :doc:`read data <read_data>`
+command to begin a simulation.  The :doc:`read_data <read_data>` command
+also describes their format.
+
+Similar to :doc:`dump <dump>` files, the data filename can contain a "*"
+wild-card character.  The "*" is replaced with the current timestep
+value.
+
+.. note::
+
+   The write-data command is not yet fully implemented in two
+   respects.  First, most pair 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, via
+   the :doc:`pair_coeff <pair_coeff>` command.  Second, a few of the :doc:`atom styles <atom_style>` (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
+:doc:`restart <restart>`, :doc:`write_restart <write_restart>`, and
+:doc:`read_restart <read_restart>` commands.  You can also convert
+binary restart files to text data files, after a simulation has run,
+using the :ref:`-r command-line switch <start_7>`.
+
+.. note::
+
+   Only limited information about a simulation is stored in a data
+   file.  For example, no information about atom :doc:`groups <group>` and
+   :doc:`fixes <fix>` are stored.  :doc:`Binary restart files <read_restart>`
+   store more information.
+
+Bond interactions (angle, etc) that have been turned off by the :doc:`fix shake <fix_shake>` or :doc:`delete_bonds <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.
+
+
+----------
+
+
+The *nocoeff* keyword requests that no force field parameters should
+be written to the data file. This can be very helpful, if one wants
+to make significant changes to the force field or if the parameters
+are read in separately anyway, e.g. from an include file.
+
+The *pair* keyword lets you specify in what format the pair
+coefficient information is written into the data file.  If the value
+is specified as *ii*\ , then one line per atom type is written, to
+specify the coefficients for each of the I=J interactions.  This means
+that no cross-interactions for I != J will be specified in the data
+file and the pair style will apply its mixing rule, as documented on
+individual :doc:`pair_style <pair_style>` doc pages.  Of course this
+behavior can be overridden in the input script after reading the data
+file, by specifying additional :doc:`pair_coeff <pair_coeff>` commands
+for any desired I,J pairs.
+
+If the value is specified as *ij*\ , then one line of coefficients is
+written for all I,J pairs where I <= J.  These coefficients will
+include any specific settings made in the input script up to that
+point.  The presence of these I != J coefficients in the data file
+will effectively turn off the default mixing rule for the pair style.
+Again, the coefficient values in the data file can can be overridden
+in the input script after reading the data file, by specifying
+additional :doc:`pair_coeff <pair_coeff>` commands for any desired I,J
+pairs.
+
+
+----------
+
+
+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
+""""""""""""""""
+
+:doc:`read_data <read_data>`, :doc:`write_restart <write_restart>`
+
+Default
+"""""""
+
+The option defaults are pair = ii.
+
+
+.. _lws: http://lammps.sandia.gov
+.. _ld: Manual.html
+.. _lc: Section_commands.html#comm