forked from lijiext/lammps
78 lines
2.5 KiB
HTML
78 lines
2.5 KiB
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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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<HR>
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<H3>improper_style ring 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 ring
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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 ring
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improper_coeff 1 8000 70.5
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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>ring</I> improper style uses the potential
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</P>
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<CENTER><IMG SRC = "Eqs/improper_ring.jpg">
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</CENTER>
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<P>where K is a prefactor, theta is the angle formed by the atoms
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specified by (i,j,k,l) indices and theta0 its equilibrium value.
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</P>
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<P>If the 4 atoms in an improper quadruplet (listed in the data file read
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by the <A HREF = "read_data.html">read_data</A> command) are ordered i,j,k,l then
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theta_<I>ijl</I> is the angle between atoms i,j and l, theta_<I>ijk</I> is the
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angle between atoms i,j and k, theta_<I>kjl</I> is the angle between atoms
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j,k, and l.
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</P>
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<P>The "ring" improper style implements the improper potential introduced
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by Destree et al., in Equation (9) of <A HREF = "#Destree">(Destree)</A>. This
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potential does not affect small amplitude vibrations but is used in an
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ad-hoc way to prevent the onset of accidentially large amplitude
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fluctuations leading to the occurrence of a planar conformation of the
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three bonds i-j, j-k and j-l, an intermediate conformation toward the
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chiral inversion of a methine carbon. In the "Impropers" section of
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data file four atoms: i, j, k and l are specified with i,j and l lying
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on the backbone of the chain and k specifying the chirality of j.
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</P>
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<P>The following coefficients must be defined for each improper type via
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the <A HREF = "improper_coeff.html">improper_coeff</A> command as in the example
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above, or in the data file or restart files read by the
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
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commands:
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</P>
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<UL><LI>K (energy/radian^2)
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<LI>theta0 (degrees)
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</UL>
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<P>theta0 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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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This improper style can only be used if LAMMPS was built with the
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USER-MISC package. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info on packages.
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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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<A NAME = "Destree"></A>
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<P><B>(Destree)</B> M. Destree, F. Laupretre, A. Lyulin, and J.-P. Ryckaert,
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J Chem Phys, 112, 9632 (2000).
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</P>
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</HTML>
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