forked from lijiext/lammps
103 lines
3.7 KiB
HTML
103 lines
3.7 KiB
HTML
<HTML>
|
|
<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>
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
<H3>fix setforce command
|
|
</H3>
|
|
<P><B>Syntax:</B>
|
|
</P>
|
|
<PRE>fix ID group-ID setforce fx fy fz keyword value ...
|
|
</PRE>
|
|
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
|
|
|
|
<LI>setforce = style name of this fix command
|
|
|
|
<LI>fx,fy,fz = force component values
|
|
|
|
<LI>any of fx,fy,fz can be a variable (see below)
|
|
|
|
<LI>zero or more keyword/value pairs may be appended to args
|
|
|
|
<LI>keyword = <I>region</I>
|
|
|
|
<PRE> <I>region</I> value = region-ID
|
|
region-ID = ID of region atoms must be in to have added force
|
|
</PRE>
|
|
|
|
</UL>
|
|
<P><B>Examples:</B>
|
|
</P>
|
|
<PRE>fix freeze indenter setforce 0.0 0.0 0.0
|
|
fix 2 edge setforce NULL 0.0 0.0
|
|
fix 2 edge setforce NULL 0.0 v_oscillate
|
|
</PRE>
|
|
<P><B>Description:</B>
|
|
</P>
|
|
<P>Set each component of force on each atom in the group to the specified
|
|
values fx,fy,fz. This erases all previously computed forces on the
|
|
atom, though additional fixes could add new forces. This command can
|
|
be used to freeze certain atoms in the simulation by zeroing their
|
|
force, assuming their initial velocity zero.
|
|
</P>
|
|
<P>Any of the fx,fy,fz values can be specified as NULL which means do not
|
|
alter the force component in that dimension.
|
|
</P>
|
|
<P>Any of the 3 quantities defining the force components can be specified
|
|
as an equal-style or atom-style <A HREF = "variable.html">variable</A>, namely <I>fx</I>,
|
|
<I>fy</I>, <I>fz</I>. If the value is a variable, it should be specified as
|
|
v_name, where name is the variable name. In this case, the variable
|
|
will be evaluated each timestep, and its value used to determine the
|
|
force component.
|
|
</P>
|
|
<P>Equal-style variables can specify formulas with various mathematical
|
|
functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
|
|
keywords for the simulation box parameters and timestep and elapsed
|
|
time. Thus it is easy to specify a time-dependent force field.
|
|
</P>
|
|
<P>Atom-style variables can specify the same formulas as equal-style
|
|
variables but can also include per-atom values, such as atom
|
|
coordinates. Thus it is easy to specify a spatially-dependent force
|
|
field with optional time-dependence as well.
|
|
</P>
|
|
<P>If the <I>region</I> keyword is used, the atom must also be in the
|
|
specified geometric <A HREF = "region.html">region</A> in order to have force added
|
|
to it.
|
|
</P>
|
|
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
|
|
</P>
|
|
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
|
|
files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
|
|
are relevant to this fix.
|
|
</P>
|
|
<P>This fix computes a global 3-vector of forces, which can be accessed
|
|
by various <A HREF = "Section_howto.html#4_15">output commands</A>. This is the
|
|
total force on the group of atoms before the forces on individual
|
|
atoms are changed by the fix. The vector values calculated by this
|
|
fix are "extensive".
|
|
</P>
|
|
<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
|
|
the <A HREF = "run.html">run</A> command.
|
|
</P>
|
|
<P>The forces due to this fix are imposed during an energy minimization,
|
|
invoked by the <A HREF = "minimize.html">minimize</A> command. You should not
|
|
specify force components with a variable that has time-dependence for
|
|
use with a minimizer, since the minimizer increments the timestep as
|
|
the iteration count during the minimization.
|
|
</P>
|
|
<P><B>Restrictions:</B> none
|
|
</P>
|
|
<P><B>Related commands:</B>
|
|
</P>
|
|
<P><A HREF = "fix_addforce.html">fix addforce</A>, <A HREF = "fix_aveforce.html">fix aveforce</A>
|
|
</P>
|
|
<P><B>Default:</B> none
|
|
</P>
|
|
</HTML>
|