improper_style inversion/harmonic command — LAMMPS documentation

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# improper_style inversion/harmonic command

## Syntax

    improper_style inversion/harmonic

## Examples

    improper_style inversion/harmonic
    improper_coeff 1 18.776340 0.000000

## Description

The inversion/harmonic improper style follows the Wilson-Decius out-of-plane angle definition and uses an harmonic potential:

\[E = K \left(\omega - \omega_0\right)^2\]

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:

- \(K\) (energy)

- \(\omega_0\) (degrees)

If \(\omega_0 = 0\) the potential term has a single minimum for the planar structure. Otherwise it has two minima at +/- \(\omega_0\), with a barrier in between.

## Restrictions

This improper style can only be used if LAMMPS was built with the MOFFF package. See the Build package doc page for more info.

## Related commands

improper_coeff

## Default

none