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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 3 X
terms used in Ei are an average over the 3 out-of-plane angles.
</P>
<P>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. X_IJKL
refers to the angle between the plane of I,J,K and the plane of J,K,L,
and the bond JK lies in both planes. Similarly for X_KJLI and X_LJIK.
Note that atom J appears in the common bonds (JI, JK, JL) of all 3 X
terms. Thus J (the 2nd atom in the quadruplet) is the atom of
symmetry in this formulation.
</P>
<P>The subscripts on the various theta's refer to different combinations
of 3 atoms (I,J,K,L) used to form a particular angle. E.g. Theta_IJL
is the angle formed by atoms I,J,L with J in the middle. 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, coefficients for the Ei formula can be specified in
eiher the input script or data file. 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 can only 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 can only be used if LAMMPS was built with the
"class2" package. See the <A HREF = "Section_start.html#2_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 = "Sun"></A>
<P><B>(Sun)</B> Sun, J Phys Chem B 102, 7338-7364 (1998).
</P>
</HTML>
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