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

doc/html/_sources/fix_momentum.txt

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+.. index:: fix momentum
+
+fix momentum command
+====================
+
+Syntax
+""""""
+
+.. parsed-literal::
+
+   fix ID group-ID momentum N keyword values ...
+
+* ID, group-ID are documented in :doc:`fix <fix>` command
+* momentum = style name of this fix command
+* N = adjust the momentum every this many timesteps
+  one or more keyword/value pairs may be appended
+* keyword = *linear* or *angular* or *rescale*
+.. parsed-literal::
+
+     *linear* values = xflag yflag zflag
+       xflag,yflag,zflag = 0/1 to exclude/include each dimension
+     *angular* values = none
+
+.. parsed-literal::
+
+     *rescale* values = none
+
+
+
+Examples
+""""""""
+
+.. parsed-literal::
+
+   fix 1 all momentum 1 linear 1 1 0
+   fix 1 all momentum 1 linear 1 1 1 rescale
+   fix 1 all momentum 100 linear 1 1 1 angular
+
+Description
+"""""""""""
+
+Zero the linear and/or angular momentum of the group of atoms every N
+timesteps by adjusting the velocities of the atoms.  One (or both) of
+the *linear* or *angular* keywords must be specified.
+
+If the *linear* keyword is used, the linear momentum is zeroed by
+subtracting the center-of-mass velocity of the group from each atom.
+This does not change the relative velocity of any pair of atoms.  One
+or more dimensions can be excluded from this operation by setting the
+corresponding flag to 0.
+
+If the *angular* keyword is used, the angular momentum is zeroed by
+subtracting a rotational component from each atom.
+
+This command can be used to insure the entire collection of atoms (or
+a subset of them) does not drift or rotate during the simulation due
+to random perturbations (e.g. :doc:`fix langevin <fix_langevin>`
+thermostatting).
+
+The *rescale* keyword enables conserving the kinetic energy of the group
+of atoms by rescaling the velocities after the momentum was removed.
+
+Note that the :doc:`velocity <velocity>` command can be used to create
+initial velocities with zero aggregate linear and/or angular momentum.
+
+Restart, fix_modify, output, run start/stop, minimize info
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+No information about this fix is written to :doc:`binary restart files <restart>`.  None of the :doc:`fix_modify <fix_modify>` options
+are relevant to this fix.  No global or per-atom quantities are stored
+by this fix for access by various :ref:`output commands <howto_15>`.  No parameter of this fix can
+be used with the *start/stop* keywords of the :doc:`run <run>` command.
+This fix is not invoked during :doc:`energy minimization <minimize>`.
+
+Restrictions
+""""""""""""
+ none
+
+Related commands
+""""""""""""""""
+
+:doc:`fix recenter <fix_recenter>`, :doc:`velocity <velocity>`
+
+**Default:** none
+
+
+.. _lws: http://lammps.sandia.gov
+.. _ld: Manual.html
+.. _lc: Section_commands.html#comm