lammps/doc/improper_cvff.txt

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improper_style cvff command :h3

[Syntax:]

improper_style cvff :pre

[Examples:]

improper_style cvff
improper_coeff 1 80.0 -1 4 :pre

[Description:]

The {cvff} improper style uses the potential

:c,image(Eqs/improper_cvff.jpg)

where phi is the Wilson out-of-plane angle.

If the 4 atoms in an improper quadruplet (listed in the data file read
by the "read_data"_read_data.html command) are ordered I,J,K,L then
the Wilson angle is between the plane of I,J,K and the plane of J,K,L.
This is essentially a dihedral angle, which is why the formula for
this improper style is the same as for "dihedral_style
harmonic"_dihedral_harmonic.html.  Alternatively, you can think of
atoms J,K,L as being in a plane, and atom I above the plane, and the
Wilson angle as a measure of how far out-of-plane I is with respect to
the other 3 atoms.

Note that defining 4 atoms to interact in this way, does not mean that
bonds necessarily exist between I-J, J-K, or K-L, as they would in a
linear dihedral.  Normally, the bonds I-J, I-K, I-L would exist for an
improper to be defined between the 4 atoms.

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:

K (energy)
d (+1 or -1)
n (0,1,2,3,4,6) :ul

[Restrictions:]

This improper style can only be used if LAMMPS was built with the
MOLECULAR package (which it is by default).  See the "Making
LAMMPS"_Section_start.html#start_3 section for more info on packages.

[Related commands:]

"improper_coeff"_improper_coeff.html

[Default:] none