Syntax:
improper_style cossq
Examples:
improper_style cossq improper_coeff 1 4.0 0.0
Description:
The cossq improper style uses the potential
where x is the improper angle, x0 is its equilibrium value, and K is a prefactor.
If the 4 atoms in an improper quadruplet (listed in the data file read by the read_data command) are ordered I,J,K,L then X is the angle between the plane of I,J,K and the plane of J,K,L. Alternatively, you can think of atoms J,K,L as being in a plane, and atom I above the plane, and X 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 command as in the example above, or in the data file or restart files read by the read_data or read_restart commands:
X0 is specified in degrees, but LAMMPS converts it to radians internally; hence the units of K are in energy/radian^2.
Restrictions:
This improper style can only be used if LAMMPS was built with the USER-MISC package. See the Making LAMMPS section for more info on packages.
Related commands:
Default: none