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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>dihedral_coeff command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>dihedral_coeff N args
</PRE>
<UL><LI>N = dihedral type (see asterisk form below)
<LI>args = coefficients for one or more dihedral types
</UL>
<P><B>Examples:</B>
</P>
<PRE>dihedral_coeff 1 80.0 1 3
dihedral_coeff * 80.0 1 3 0.5
dihedral_coeff 2* 80.0 1 3 0.5
</PRE>
<P><B>Description:</B>
</P>
<HR>
<P>Specify the dihedral force field coefficients for one or more dihedral types.
The number and meaning of the coefficients depends on the dihedral style.
Dihedral coefficients can also be set in the data file read by the
<A HREF = "read_data.html">read_data</A> command or in a restart file.
</P>
<P>N can be specified in one of two ways. An explicit numeric value can
be used, as in the 1st example above. Or a wild-card asterisk can be
used to set the coefficients for multiple dihedral types. This takes the
form "*" or "*n" or "n*" or "m*n". If N = the number of dihedral types,
then an asterisk with no numeric values means all types from 1 to N. A
leading asterisk means all types from 1 to n (inclusive). A trailing
asterisk means all types from n to N (inclusive). A middle asterisk
means all types from m to n (inclusive).
</P>
<P>Note that using a dihedral_coeff command can override a previous setting
for the same dihedral type. For example, these commands set the coeffs
for all dihedral types, then overwrite the coeffs for just dihedral type 2:
</P>
<PRE>dihedral_coeff * 80.0 1 3
dihedral_coeff 2 200.0 1 3
</PRE>
<P>A line in a data file that specifies dihedral coefficients uses the exact
same format as the arguments of the dihedral_coeff command in an input
script, except that wild-card asterisks should not be used since
coefficients for all N types must be listed in the file. For example,
under the "Dihedral Coeffs" section of a data file, the line that
corresponds to the 1st example above would be listed as
</P>
<PRE>1 80.0 1 3
</PRE>
<P>The <A HREF = "dihedral_class2.html">dihedral_style class2</A> is an exception to
this rule, in that an additional argument is used in the input script
to allow specification of the cross-term coefficients. See its doc
page for details.
</P>
<P>IMPORTANT NOTE: When comparing the formulas and coefficients for
various LAMMPS dihedral styles with dihedral equations defined by
other force fields, note that some force field implementations
divide/multiply the energy prefactor <I>K</I> by the multiple number of
torsions that contain the J-K bond in an I-J-K-L torsion. LAMMPS does
not do this, i.e. the listed dihedral equation applies to each
individual dihedral. Thus you need to define <I>K</I> appropriately to
account for this difference if necessary.
</P>
<HR>
<P>Here is an alphabetic list of dihedral styles defined in LAMMPS. Click on
the style to display the formula it computes and coefficients
specified by the associated <A HREF = "dihedral_coeff.html">dihedral_coeff</A> command.
</P>
<P>Note that there are also additional dihedral styles submitted by users
which are included in the LAMMPS distribution. The list of these with
links to the individual styles are given in the dihedral section of
<A HREF = "Section_commands.html#cmd_5">this page</A>.
</P>
<UL><LI><A HREF = "dihedral_none.html">dihedral_style none</A> - turn off dihedral interactions
<LI><A HREF = "dihedral_hybrid.html">dihedral_style hybrid</A> - define multiple styles of dihedral interactions
</UL>
<UL><LI><A HREF = "dihedral_charmm.html">dihedral_style charmm</A> - CHARMM dihedral
<LI><A HREF = "dihedral_class2.html">dihedral_style class2</A> - COMPASS (class 2) dihedral
<LI><A HREF = "dihedral_harmonic.html">dihedral_style harmonic</A> - harmonic dihedral
<LI><A HREF = "dihedral_helix.html">dihedral_style helix</A> - helix dihedral
<LI><A HREF = "dihedral_multi_harmonic.html">dihedral_style multi/harmonic</A> - multi-harmonic dihedral
<LI><A HREF = "dihedral_opls.html">dihedral_style opls</A> - OPLS dihedral
</UL>
<HR>
<P><B>Restrictions:</B>
</P>
<P>This command must come after the simulation box is defined by a
<A HREF = "read_data.html">read_data</A>, <A HREF = "read_restart.html">read_restart</A>, or
<A HREF = "create_box.html">create_box</A> command.
</P>
<P>A dihedral style must be defined before any dihedral coefficients are
set, either in the input script or in a data file.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "dihedral_style.html">dihedral_style</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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