2006-09-22 00:22:34 +08:00
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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>
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<H3>improper_style class2 command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>improper_style class2
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>improper_style class2
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improper_coeff 1 100.0 0
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>class2</I> improper style uses the potential
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</P>
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<CENTER><IMG SRC = "Eqs/improper_class2.jpg">
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</CENTER>
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2007-06-28 23:07:55 +08:00
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<P>where Ei is the improper term and Eaa is an angle-angle term. The 3 X
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2007-06-29 00:06:08 +08:00
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terms in Ei are an average over 3 out-of-plane angles.
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2007-06-28 23:07:55 +08:00
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</P>
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<P>The 4 atoms in an improper quadruplet (listed in the data file read by
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the <A HREF = "read_data.html">read_data</A> command) are ordered I,J,K,L. X_IJKL
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refers to the angle between the plane of I,J,K and the plane of J,K,L,
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and the bond JK lies in both planes. Similarly for X_KJLI and X_LJIK.
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Note that atom J appears in the common bonds (JI, JK, JL) of all 3 X
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terms. Thus J (the 2nd atom in the quadruplet) is the atom of
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2007-06-28 23:25:31 +08:00
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symmetry in the 3 X angles.
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2007-06-28 23:07:55 +08:00
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</P>
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<P>The subscripts on the various theta's refer to different combinations
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of 3 atoms (I,J,K,L) used to form a particular angle. E.g. Theta_IJL
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is the angle formed by atoms I,J,L with J in the middle. Theta1,
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2007-06-28 23:25:31 +08:00
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theta2, theta3 are the equilibrium positions of those angles. Again,
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atom J (the 2nd atom in the quadruplet) is the atom of symmetry in the
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theta angles, since it is always the center atom.
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</P>
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2007-06-29 05:59:10 +08:00
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<P>Since atom J is the atom of symmetry, normally the bonds J-I, J-K, J-L
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would exist for an improper to be defined between the 4 atoms, but
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this is not required.
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2006-09-22 00:22:34 +08:00
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</P>
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2006-09-28 03:12:31 +08:00
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<P>See <A HREF = "#Sun">(Sun)</A> for a description of the COMPASS class2 force field.
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</P>
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2006-09-22 00:22:34 +08:00
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<P>The following coefficients must be defined for each improper type via the
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<A HREF = "improper_coeff.html">improper_coeff</A> command as in the example above, or in
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the data file or restart files read by the <A HREF = "read_data.html">read_data</A>
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or <A HREF = "read_restart.html">read_restart</A> commands:
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</P>
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2007-06-25 22:36:36 +08:00
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<P>For this style, coefficients for the Ei formula can be specified in
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2008-03-01 09:13:20 +08:00
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either the input script or data file. These are the 2 coefficients:
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2006-09-22 00:22:34 +08:00
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</P>
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<UL><LI>K (energy/radian^2)
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<LI>X0 (degrees)
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</UL>
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<P>X0 is specified in degrees, but LAMMPS converts it to radians
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internally; hence the units of K are in energy/radian^2.
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</P>
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2007-06-25 22:36:36 +08:00
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<P>Coefficients for the Eaa formula can only be specified in the data
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file. For the Eaa formula, the coefficients are listed under a
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2006-09-22 00:22:34 +08:00
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"AngleAngle Coeffs" heading and each line lists 6 coefficients:
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</P>
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<UL><LI>M1 (energy/distance)
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<LI>M2 (energy/distance)
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<LI>M3 (energy/distance)
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<LI>theta1 (degrees)
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<LI>theta2 (degrees)
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<LI>theta3 (degrees)
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</UL>
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<P>The theta values are specified in degrees, but LAMMPS converts them to
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radians internally; hence the units of M are in energy/radian^2.
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</P>
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<P><B>Restrictions:</B>
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</P>
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2007-06-25 22:36:36 +08:00
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<P>This improper style can only be used if LAMMPS was built with the
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"class2" package. See the <A HREF = "Section_start.html#2_3">Making LAMMPS</A>
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section for more info on packages.
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2006-09-22 00:22:34 +08:00
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "improper_coeff.html">improper_coeff</A>
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</P>
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<P><B>Default:</B> none
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</P>
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2006-09-28 03:12:31 +08:00
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<HR>
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<A NAME = "Sun"></A>
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<P><B>(Sun)</B> Sun, J Phys Chem B 102, 7338-7364 (1998).
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</P>
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2006-09-22 00:22:34 +08:00
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</HTML>
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