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improper_style command

Syntax:

improper_style style 

Examples:

improper_style harmonic
improper_style cvff 

Description:

Set the formula LAMMPS will use to compute improper interactions between quadruplets (trigonal centers) of atoms. The list of atom quadruplets is specified in the data or restart file and is read in by a read_data or read_restart command. The coefficients for the formula for each improper type can also be specified in those files or by the improper_coeff command.

A style of none means improper forces are not computed, even if impropers are defined.


The class2 style uses the potential

where Ei is the improper term and Eaa is an angle-angle term. The chi used in Ei is an average over 3 possible chi orientations. The subscripts on the various theta's refer to different combinations of atoms i,j,k,l used to form the angle; theta1, theta2, theta3 are the equilibrium positions of those angles. K, Mn, chi0, theta1, theta2, and theta3 are coefficients defined for each improper type.


The cvff style uses the potential

where phi is the Wilson out-of-plane angle. K, d, and n are coefficients defined for each improper type.


The harmonic style uses the potential

where X is the improper angle, X0 is its equilibrium value, and K is a prefactor. Note that the usual 1/2 factor is included in K. K and X0 are coefficients defined for each improper type.

Restrictions:

Improper styles can only be set for atom_style choices that allow impropers to be defined.

Improper styles are part of the "molecular" package. They are only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

Related commands:

improper_coeff

Default:

improper_style none