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dihedral_cosine_shift_exp.html
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rLAMMPS lammps
dihedral_cosine_shift_exp.html
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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_style cosine/shift/exp command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
dihedral_style cosine/shift/exp
</PRE>
<P><B>
Examples:
</B>
</P>
<PRE>
dihedral_style cosine/shift/exp
dihedral_coeff 1 10.0 45.0 2.0
</PRE>
<P><B>
Description:
</B>
</P>
<P>
The
<I>
cosine/shift/exp
</I>
dihedral style uses the potential
</P>
<CENTER><IMG
SRC =
"Eqs/dihedral_cosine_shift_exp.jpg"
>
</CENTER>
<P>
where Umin, theta, and a are defined for each dihedral type.
</P>
<P>
The potential is bounded between [-Umin:0] and the minimum is located
at the angle theta0. The a parameter can be both positive or negative
and is used to control the spring constant at the equilibrium.
</P>
<P>
The spring constant is given by k=a exp(a) Umin/ [2 (Exp(a)-1)].
For a>3 k/Umin = a/2 to better than 5% relative error. For negative
values of the a parameter, the spring constant is essentially zero,
and anharmonic terms takes over. The potential is furthermore well
behaved in the limit a->0, where it has been implemented to linear
order in a for a
< 0
.
001
.
</
P
>
<P>
The following coefficients must be defined for each dihedral type via
the
<A
HREF =
"dihedral_coeff.html"
>
dihedral_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>
<UL><LI>
umin (energy)
<LI>
theta (angle)
<LI>
A (real number)
</UL>
<P><B>
Restrictions:
</B>
</P>
<P>
This dihedral style can only be used if LAMMPS was built with the
"user-misc" 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 =
"dihedral_coeff.html"
>
dihedral_coeff
</A>
,
<A
HREF =
"angle_cosineshiftexp.html"
>
angle_cosineshiftexp
</A>
</P>
<P><B>
Default:
</B>
none
</P>
</HTML>
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