"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

improper_style class2 command :h3

[Syntax:]

improper_style class2 :pre

[Examples:]

improper_style class2
improper_coeff 1 100.0 0 :pre

[Description:]

The {class2} improper style uses the potential

:c,image(Eqs/improper_class2.jpg)

where Ei is the improper term and Eaa is an angle-angle term.  The chi
used in Ei is an average over 3 possible chi orientations.  The
subscripts on the various theta's refer to different combinations of
atoms i,j,k,l used to form the angle; theta1, theta2, theta3 are the
equilibrium positions of those angles.

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

The following coefficients must be defined for each improper type via the
"improper_coeff"_improper_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:

For this style, only coefficients for the Ei formula can be specified
in the input script.  These are the 2 coefficients:

K (energy/radian^2)
X0 (degrees) :ul

X0 is specified in degrees, but LAMMPS converts it to radians
internally; hence the units of K are in energy/radian^2.

Coefficients for the Eaa formula must be specified in the data file.
For the Eaa formula, the coefficients are listed under a
"AngleAngle Coeffs" heading and each line lists 6 coefficients:

M1 (energy/distance)
M2 (energy/distance)
M3 (energy/distance)
theta1 (degrees)
theta2 (degrees)
theta3 (degrees) :ul

The theta values are specified in degrees, but LAMMPS converts them to
radians internally; hence the units of M are in energy/radian^2.

[Restrictions:]

This improper style is part of the "class2" package.  It is only
enabled if LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#2_2 section for more info.

[Related commands:]

"improper_coeff"_improper_coeff.html

[Default:] none

:line

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