improper_style inversion/harmonic command
improper_style inversion/harmonic improper_coeff 1 18.776340 0.000000
The inversion/harmonic improper style follows the Wilson-Decius out-of-plane angle definition and uses an harmonic potential:
where K is the force constant and omega is the angle evaluated for all three axis-plane combinations centered around the atom I. For the IL axis and the IJK plane omega looks as follows:
Note that the inversion/harmonic angle term evaluation differs to the improper_umbrella due to the cyclic evaluation of all possible angles omega.
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:
If omega0 = 0 the potential term has a minimum for the planar structure. Otherwise it has two minima at +/- omega0, with a barrier in between.
This improper style can only be used if LAMMPS was built with the USER-MOFFF package. See the Build package doc page for more info.