# improper_style harmonic command¶

## Syntax¶

improper_style amoeba


## Examples¶

improper_style amoeba
improper_coeff 1 49.6


## Description¶

The amoeba improper style uses the potential

$E = K (\chi)^2$

where $$\chi$$ is the improper angle and $$K$$ is a prefactor. Note that the usual 1/2 factor is included in $$K$$.

This formula seems like a simplified version of the formula for the improper_style harmonic command with $$\chi_0$$ = 0.0. However the computation of the angle $$\chi$$ is done differently to match how the Tinker MD code computes its out-of-plane improper for the AMOEBA and HIPPO force fields. See the Howto amoeba doc page for more information about the implementation of AMOEBA and HIPPO in LAMMPS.

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 atoms I,K,L are considered to lie in a plane and atom J is out-of-place. The angle $$\chi_0$$ is computed as the Allinger angle which is defined as the angle between the plane of I,K,L, and the vector from atom I to atom J.

The following coefficient 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)

Note that the angle $$\chi$$ is computed in radians; hence $$K$$ is effectively energy per radian^2.

## Restrictions¶

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

none