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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 class2 command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>improper_style class2
</PRE>
<P><B>Examples:</B>
</P>
<PRE>improper_style class2
improper_coeff 1 100.0 0
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>class2</I> improper style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/improper_class2.jpg">
</CENTER>
<P>where Ei is the improper term and Eaa is an angle-angle term. The chi
used in Ei is an average over 3 possible chi orientations. The
subscripts on the various theta's refer to different combinations of
atoms i,j,k,l used to form the angle; theta1, theta2, theta3 are the
equilibrium positions of those angles.
</P>
<P>See <A HREF = "#Sun">(Sun)</A> for a description of the COMPASS class2 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>
<P>For this style, only coefficients for the Ei formula can be specified
in the input script. These are the 2 coefficients:
</P>
<UL><LI>K (energy/radian^2)
<LI>X0 (degrees)
</UL>
<P>X0 is specified in degrees, but LAMMPS converts it to radians
internally; hence the units of K are in energy/radian^2.
</P>
<P>Coefficients for the Eaa formula must be specified in the data file.
For the Eaa formula, the coefficients are listed under a
"AngleAngle Coeffs" heading and each line lists 6 coefficients:
</P>
<UL><LI>M1 (energy/distance)
<LI>M2 (energy/distance)
<LI>M3 (energy/distance)
<LI>theta1 (degrees)
<LI>theta2 (degrees)
<LI>theta3 (degrees)
</UL>
<P>The theta values are specified in degrees, but LAMMPS converts them to
radians internally; hence the units of M are in energy/radian^2.
</P>
<P><B>Restrictions:</B>
</P>
<P>This improper style is part of the "class2" package. It is only
enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_2">Making
LAMMPS</A> section for more info.
</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 = "Sun"></A>
<P><B>(Sun)</B> Sun, J Phys Chem B 102, 7338-7364 (1998).
</P>
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