lammps/doc/angle_class2.txt

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angle_style class2 command :h3
angle_style class2/omp command :h3

[Syntax:]

angle_style class2 :pre

[Examples:]

angle_style class2
angle_coeff * 75.0
angle_coeff 1 bb 10.5872 1.0119 1.5228
angle_coeff * ba 3.6551 24.895 1.0119 1.5228 :pre

[Description:]

The {class2} angle style uses the potential

:c,image(Eqs/angle_class2.jpg)

where Ea is the angle term, Ebb is a bond-bond term, and Eba is a
bond-angle term.  Theta0 is the equilibrium angle and r1 and r2 are
the equilibrium bond lengths.

See "(Sun)"_#Sun for a description of the COMPASS class2 force field.

Coefficients for the Ea, Ebb, and Eba formulas must be defined for
each angle type via the "angle_coeff"_angle_coeff.html command as in
the example above, or in the data file or restart files read by the
"read_data"_read_data.html or "read_restart"_read_restart.html
commands.

These are the 4 coefficients for the Ea formula:

theta0 (degrees)
K2 (energy/radian^2)
K3 (energy/radian^3)
K4 (energy/radian^4) :ul

Theta0 is specified in degrees, but LAMMPS converts it to radians
internally; hence the units of the various K are in per-radian.

For the Ebb formula, each line in a "angle_coeff"_angle_coeff.html
command in the input script lists 4 coefficients, the first of which
is "bb" to indicate they are BondBond coefficients.  In a data file,
these coefficients should be listed under a "BondBond Coeffs" heading
and you must leave out the "bb", i.e. only list 3 coefficients after
the angle type.

bb
M (energy/distance^2)
r1 (distance)
r2 (distance) :ul

For the Eba formula, each line in a "angle_coeff"_angle_coeff.html
command in the input script lists 5 coefficients, the first of which
is "ba" to indicate they are BondAngle coefficients.  In a data file,
these coefficients should be listed under a "BondAngle Coeffs" heading
and you must leave out the "ba", i.e. only list 4 coefficients after
the angle type.

ba
N1 (energy/distance^2)
N2 (energy/distance^2)
r1 (distance)
r2 (distance) :ul

The theta0 value in the Eba formula is not specified, since it is the
same value from the Ea formula.

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Styles with a {cuda}, {gpu}, {intel}, {kk}, {omp}, or {opt} 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 "Section_accelerate"_Section_accelerate.html
of the manual.  The accelerated styles take the same arguments and
should produce the same results, except for round-off and precision
issues.

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 "Making
LAMMPS"_Section_start.html#start_3 section for more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the "-suffix command-line
switch"_Section_start.html#start_7 when you invoke LAMMPS, or you can
use the "suffix"_suffix.html command in your input script.

See "Section_accelerate"_Section_accelerate.html of the manual for
more instructions on how to use the accelerated styles effectively.

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[Restrictions:]

This angle style can only be used if LAMMPS was built with the CLASS2
package.  See the "Making LAMMPS"_Section_start.html#start_3 section
for more info on packages.

[Related commands:]

"angle_coeff"_angle_coeff.html

[Default:] none

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:link(Sun)
[(Sun)] Sun, J Phys Chem B 102, 7338-7364 (1998).