Syntax:
improper_style class2
Examples:
improper_style class2 improper_coeff 1 100.0 0
Description:
The class2 improper 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.
See (Sun) for a description of the COMPASS class2 force field.
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:
For this style, only coefficients for the Ei formula can be specified in the input script. These are the 2 coefficients:
X0 is specified in degrees, but LAMMPS converts it to radians internally; hence the units of K are in energy/radian^2.
Coefficients for the Eaa formula must be specified in the data file. For the Eaa formula, the coefficients are listed under a "AngleAngle Coeffs" heading and each line lists 6 coefficients:
The theta values are specified in degrees, but LAMMPS converts them to radians internally; hence the units of M are in energy/radian^2.
Restrictions:
This improper style is part of the "class2" package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.
Related commands:
Default: none
(Sun) Sun, J Phys Chem B 102, 7338-7364 (1998).