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<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>improper_style umbrella command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>improper_style umbrella
</PRE>
<P><B>Examples:</B>
</P>
<PRE>improper_style umbrella
improper_coeff 1 100.0 180.0
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>umbrella</I> improper style uses the following potential, which is
commonly referred to as a classic inversion and used in the
<A HREF = "Section_howto.html#howto_4">DREIDING</A> force field:
</P>
<CENTER><IMG SRC = "Eqs/improper_umbrella.jpg">
</CENTER>
<P>where K is the force constant and omega is the angle between the IL
axis and the IJK plane:
</P>
<CENTER><IMG SRC = "Eqs/umbrella.jpg">
</CENTER>
<P>If omega0 = 0 the potential term has a minimum for the planar
structure. Otherwise it has two minima at +/- omega0, with a barrier
in between.
</P>
<P>See <A HREF = "#Mayo">(Mayo)</A> for a description of the DREIDING force field.
</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>omega0 (degrees)
</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>
<HR>
<A NAME = "Mayo"></A>
<P><B>(Mayo)</B> Mayo, Olfason, Goddard III, J Phys Chem, 94, 8897-8909
(1990),
</P>
</HTML>
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