forked from lijiext/lammps
104 lines
3.8 KiB
HTML
104 lines
3.8 KiB
HTML
<HTML>
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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 aveforce 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 aveforce fx fy fz keyword value ...
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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>aveforce = style name of this fix command
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<LI>fx,fy,fz = force component values (force units)
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<LI>any of fx,fy,fz can be a variable (see below)
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<LI>zero or more keyword/value pairs may be appended to args
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<LI>keyword = <I>region</I>
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<PRE> <I>region</I> value = region-ID
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region-ID = ID of region atoms must be in to have added force
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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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<PRE>fix pressdown topwall aveforce 0.0 -1.0 0.0
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fix 2 bottomwall aveforce NULL -1.0 0.0 region top
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fix 2 bottomwall aveforce NULL -1.0 v_oscillate region top
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Apply an additional external force to a group of atoms in such a way
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that every atom experiences the same force. This is useful for
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pushing on wall or boundary atoms so that the structure of the wall
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does not change over time.
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</P>
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<P>The existing force is averaged for the group of atoms, component by
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component. The actual force on each atom is then set to the average
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value plus the component specified in this command. This means each
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atom in the group receives the same force.
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</P>
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<P>Any of the fx,fy,fz values can be specified as NULL which means the
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force in that dimension is not changed. Note that this is not the
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same as specifying a 0.0 value, since that sets all forces to the same
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average value without adding in any additional force.
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</P>
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<P>Any of the 3 quantities defining the force components can be specified
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as an equal-style <A HREF = "variable.html">variable</A>, namely <I>fx</I>, <I>fy</I>, <I>fz</I>.
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If the value is a variable, it should be specified as v_name, where
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name is the variable name. In this case, the variable will be
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evaluated each timestep, and its value used to determine the average
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force.
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</P>
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<P>Equal-style variables can specify formulas with various mathematical
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functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent average force.
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</P>
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<P>If the <I>region</I> keyword is used, the atom must also be in the
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specified geometric <A HREF = "region.html">region</A> in order to have force added
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to it.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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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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are relevant to this fix.
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</P>
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<P>This fix computes a global 3-vector of forces, which can be accessed
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by various <A HREF = "Section_howto.html#4_15">output commands</A>. This is the
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total force on the group of atoms before the forces on individual
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atoms are changed by the fix. The vector values calculated by this
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fix are "extensive".
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command.
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</P>
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<P>The forces due to this fix are imposed during an energy minimization,
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invoked by the <A HREF = "minimize.html">minimize</A> command. You should not
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specify force components with a variable that has time-dependence for
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use with a minimizer, since the minimizer increments the timestep as
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the iteration count during the minimization.
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</P>
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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_setforce.html">fix setforce</A>, <A HREF = "fix_addforce.html">fix addforce</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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