2006-09-22 00:22:34 +08:00
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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>fix momentum command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID momentum N keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>momentum = style name of this fix command
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2006-10-03 04:21:36 +08:00
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<LI>N = adjust the momentum every this many timesteps
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2007-07-03 04:04:44 +08:00
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one or more keyword/value pairs may be appended
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2006-09-22 00:22:34 +08:00
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<LI>keyword = <I>linear</I> or <I>angular</I>
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<PRE> <I>linear</I> values = xflag yflag zflag
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xflag,yflag,zflag = 0/1 to exclude/include each dimension
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<I>angular</I> values = none
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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2006-10-03 04:21:36 +08:00
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<PRE>fix 1 all momentum 1 linear 1 1 0
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fix 1 all momentum 100 linear 1 1 1 angular
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2006-09-22 00:22:34 +08:00
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Zero the linear and/or angular momentum of the group of atoms every N
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2006-10-03 04:21:36 +08:00
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timesteps by adjusting the velocities of the atoms. One (or both) of
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the <I>linear</I> or <I>angular</I> keywords must be specified.
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2006-09-22 00:22:34 +08:00
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</P>
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<P>If the <I>linear</I> keyword is used, the linear momentum is zeroed by
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subtracting the center-of-mass velocity of the group from each atom.
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This does not change the relative velocity of any pair of atoms. One
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or more dimensions can be excluded from this operation by setting the
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corresponding flag to 0.
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</P>
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<P>If the <I>angular</I> keyword is used, the angular momentum is zeroed by
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subtracting a rotational component from each atom.
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</P>
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<P>This command can be used to insure the entire collection of atoms (or
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a subset of them) does not drift or rotate during the simulation due
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to random perturbations (e.g. <A HREF = "fix_langevin.html">fix langevin</A>
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thermostatting).
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</P>
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<P>Note that the <A HREF = "velocity.html">velocity</A> command can be used to create
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2006-10-03 04:21:36 +08:00
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initial velocities with zero aggregate linear and/or angular momentum.
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2006-09-22 00:22:34 +08:00
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</P>
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2007-10-11 06:28:11 +08:00
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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2007-06-26 08:03:39 +08:00
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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2010-04-03 00:51:06 +08:00
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are relevant to this fix. No global or per-atom quantities are stored
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2011-08-26 01:01:01 +08:00
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by this fix for access by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. No parameter of this fix can
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be used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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2007-10-11 06:28:11 +08:00
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This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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2007-06-26 08:03:39 +08:00
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</P>
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2006-09-22 00:22:34 +08:00
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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 = "fix_recenter.html">fix recenter</A>, <A HREF = "velocity.html">velocity</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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