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rLAMMPS lammps
improper_class2.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>
improper_style class2 command
</H3>
<H3>
improper_style class2/omp command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
improper_style class2
</PRE>
<P><B>
Examples:
</B>
</P>
<PRE>
improper_style class2
improper_coeff 1 100.0 0
improper_coeff * aa 0.0 0.0 0.0 115.06 130.01 115.06
</PRE>
<P><B>
Description:
</B>
</P>
<P>
The
<I>
class2
</I>
improper style uses the potential
</P>
<CENTER><IMG
SRC =
"Eqs/improper_class2.jpg"
>
</CENTER>
<P>
where Ei is the improper term and Eaa is an angle-angle term. The 3 X
terms in Ei are an average over 3 out-of-plane angles.
</P>
<P>
The 4 atoms in an improper quadruplet (listed in the data file read by
the
<A
HREF =
"read_data.html"
>
read_data
</A>
command) are ordered I,J,K,L. X_IJKL
refers to the angle between the plane of I,J,K and the plane of J,K,L,
and the bond JK lies in both planes. Similarly for X_KJLI and X_LJIK.
Note that atom J appears in the common bonds (JI, JK, JL) of all 3 X
terms. Thus J (the 2nd atom in the quadruplet) is the atom of
symmetry in the 3 X angles.
</P>
<P>
The subscripts on the various theta's refer to different combinations
of 3 atoms (I,J,K,L) used to form a particular angle. E.g. Theta_IJL
is the angle formed by atoms I,J,L with J in the middle. Theta1,
theta2, theta3 are the equilibrium positions of those angles. Again,
atom J (the 2nd atom in the quadruplet) is the atom of symmetry in the
theta angles, since it is always the center atom.
</P>
<P>
Since atom J is the atom of symmetry, normally the bonds J-I, J-K, J-L
would exist for an improper to be defined between the 4 atoms, but
this is not required.
</P>
<P>
See
<A
HREF =
"#Sun"
>
(Sun)
</A>
for a description of the COMPASS class2 force field.
</P>
<P>
Coefficients for the Ei and Eaa formulas must be defined for each
improper type via the
<A
HREF =
"improper_coeff.html"
>
improper_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>
<P>
These are the 2 coefficients for the Ei formula:
</P>
<UL><LI>
K (energy/radian^2)
<LI>
X0 (degrees)
</UL>
<P>
X0 is specified in degrees, but LAMMPS converts it to radians
internally; hence the units of K are in energy/radian^2.
</P>
<P>
For the Eaa formula, each line in a
<A
HREF =
"improper_coeff.html"
>
improper_coeff
</A>
command in the input script lists
7 coefficients, the first of which is "aa" to indicate they are
AngleAngle coefficients. In a data file, these coefficients should be
listed under a "AngleAngle Coeffs" heading and you must leave out the
"aa", i.e. only list 6 coefficients after the improper type.
</P>
<UL><LI>
aa
<LI>
M1 (energy/distance)
<LI>
M2 (energy/distance)
<LI>
M3 (energy/distance)
<LI>
theta1 (degrees)
<LI>
theta2 (degrees)
<LI>
theta3 (degrees)
</UL>
<P>
The theta values are specified in degrees, but LAMMPS converts them to
radians internally; hence the units of M are in energy/radian^2.
</P>
<HR>
<P>
Styles with a
<I>
cuda
</I>
,
<I>
gpu
</I>
,
<I>
intel
</I>
,
<I>
kk
</I>
,
<I>
omp
</I>
, or
<I>
opt
</I>
suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed in
<A
HREF =
"Section_accelerate.html"
>
Section_accelerate
</A>
of the manual. The accelerated styles take the same arguments and
should produce the same results, except for round-off and precision
issues.
</P>
<P>
These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
KOKKOS, USER-OMP and OPT packages, respectively. They are only
enabled if LAMMPS was built with those packages. See the
<A
HREF =
"Section_start.html#start_3"
>
Making
LAMMPS
</A>
section for more info.
</P>
<P>
You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the
<A
HREF =
"Section_start.html#start_7"
>
-suffix command-line
switch
</A>
when you invoke LAMMPS, or you can
use the
<A
HREF =
"suffix.html"
>
suffix
</A>
command in your input script.
</P>
<P>
See
<A
HREF =
"Section_accelerate.html"
>
Section_accelerate
</A>
of the manual for
more instructions on how to use the accelerated styles effectively.
</P>
<HR>
<P><B>
Restrictions:
</B>
</P>
<P>
This improper style can only be used if LAMMPS was built with the
CLASS2 package. See the
<A
HREF =
"Section_start.html#start_3"
>
Making LAMMPS
</A>
section for more info on packages.
</P>
<P><B>
Related commands:
</B>
</P>
<P><A
HREF =
"improper_coeff.html"
>
improper_coeff
</A>
</P>
<P><B>
Default:
</B>
none
</P>
<HR>
<A
NAME =
"Sun"
></A>
<P><B>
(Sun)
</B>
Sun, J Phys Chem B 102, 7338-7364 (1998).
</P>
</HTML>
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