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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>dihedral_style command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>dihedral_style style
</PRE>
<UL><LI>style = <I>none</I> or <I>hybrid</I> or <I>charmm</I> or <I>class2</I> or <I>harmonic</I> or <I>helix</I> or <I>multi/harmonic</I> or <I>opls</I>
</UL>
<P><B>Examples:</B>
</P>
<PRE>dihedral_style harmonic
dihedral_style multi/harmonic
dihedral_style hybrid harmonic charmm
</PRE>
<P><B>Description:</B>
</P>
<P>Set the formula(s) LAMMPS uses to compute dihedral interactions
between quadruplets of atoms, which remain in force for the duration
of the simulation. The list of dihedral quadruplets is read in by a
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A> command
from a data or restart file.
</P>
<P>Hybrid models where dihedrals are computed using different dihedral
potentials can be setup using the <I>hybrid</I> dihedral style.
</P>
<P>The coefficients associated with a dihedral style can be specified in a
data or restart file or via the <A HREF = "dihedral_coeff.html">dihedral_coeff</A> command.
</P>
<P>In the formulas listed for each dihedral style, <I>phi</I> is the torsional
angle defined by the quadruplet of atoms.
</P>
<P>Note that when both a dihedral and pair style is defined, the
<A HREF = "special_bond.html">special_bond</A> command often needs to be used to
turn off (or weight) the pairwise interactions that would otherwise
exist between the 4 bonded atoms.
</P>
<P>Here are some important points to take note of when defining the
LAMMPS dihedral coefficients in the formulas that follow so that they
are compatible with other force fields:
</P>
<UL><LI>The LAMMPS convention is that the trans position = 180 degrees, while
in some force fields trans = 0 degrees.
<LI>Some force fields reverse the sign convention on <I>d</I>.
<LI>Some force fields divide/multiply <I>K</I> by the number of multiple
torsions that contain the j-k bond in an i-j-k-l torsion.
<LI>Some force fields let <I>n</I> be positive or negative which corresponds to
<I>d</I> = 1 or -1 for the harmonic style.
</UL>
<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>
<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>Dihedral styles can only be set for atom styles that allow dihedrals
to be defined.
</P>
<P>Dihedral styles are part of the "molecular" package or other packages
as noted in their documentation. They are 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 = "dihedral_coeff.html">dihedral_coeff</A>
</P>
<P><B>Default:</B>
</P>
<P>dihedral_style none
</P>
</HTML>
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