improper_style amoeba command
improper_style amoeba improper_coeff 1 49.6
The amoeba improper style uses the potential
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:
Note that the angle \(\chi\) is computed in radians; hence \(K\) is effectively energy per radian^2.
This improper style can only be used if LAMMPS was built with the AMOEBA package. See the Build package doc page for more info.