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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>
</CENTER>
<HR>
<H3>improper_style cvff command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>improper_style cvff
</PRE>
<P><B>Examples:</B>
</P>
<PRE>improper_style cvff
improper_coeff 1 80.0 -1 4
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>cvff</I> improper style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/improper_cvff.jpg">
</CENTER>
<P>where phi is the Wilson out-of-plane angle.
</P>
<P>If the 4 atoms in an improper quadruplet (listed in the data file read
by the <A HREF = "read_data.html">read_data</A> command) are ordered I,J,K,L then
the Wilson angle is between the plane of I,J,K and the plane of J,K,L.
This is essentially a dihedral angle, which is why the formula for
this improper style is the same as for <A HREF = "dihedral_harmonic.html">dihedral_style
harmonic</A>. Alternatively, you can think of
atoms J,K,L as being in a plane, and atom I above the plane, and the
Wilson angle as a measure of how far out-of-plane I is with respect to
the other 3 atoms.
</P>
<P>Note that defining 4 atoms to interact in this way, does not mean that
bonds necessarily exist between I-J, J-K, or K-L, as they would in a
linear dihedral. Normally, the bonds I-J, I-K, I-L would exist for an
improper to be defined between the 4 atoms.
</P>
<P>The following coefficients must be defined for each improper type via
the <A HREF = "improper_coeff.html">improper_coeff</A> command as in the example
above, or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands:
</P>
<UL><LI>K (energy)
<LI>d (+1 or -1)
<LI>n (0,1,2,3,4,6)
</UL>
<P><B>Restrictions:</B>
</P>
<P>This improper style can only be used if LAMMPS was built with the
MOLECULAR package (which it is by default). See the <A HREF = "Section_start.html#start_3">Making
LAMMPS</A> section for more info on packages.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "improper_coeff.html">improper_coeff</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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