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dihedral_coeff command

Syntax:

dihedral_coeff N args 

Examples:

dihedral_coeff 1 80.0 1 3
dihedral_coeff * 80.0 1 3 0.5
dihedral_coeff 2* 80.0 1 3 0.5 

Description:

Specify the force field coefficients for one or more dihedral types. The number and meaning of the coefficients depends on the dihedral style. As described below, dihedral coefficients can also be set in the data file read by the read_data command or in a restart file.

N can be specified in one of two ways. An explicit numeric value can be used, as in the 1st example above. Or a wild-card asterik can be used to set the coefficients for multiple dihedral types. This takes the form "*" or "*n" or "n*" or "m*n". If N = the number of dihedral types, then an asterik with no numeric values means all types from 1 to N. A leading asterik means all types from 1 to n (inclusive). A trailing asterik means all types from n to N (inclusive). A middle asterik means all types from m to n (inclusive).

Note that using 2 dihedral_coeff commands for the same dihedral type is perfectly valid. For example, these commands set the coeffs for all dihedral types, then overwrite the coeffs for just dihedral type 2:

dihedral_coeff * 80.0 1 3
dihedral_coeff 2 200.0 1 3 

A line in a data file that specifies dihedral coefficients uses the exact same format as the arguments of the dihedral_coeff command in an input script, except that wild-card asteriks should not be used since coefficients for all N types are listed in the file. For example, under the "Dihedral Coeffs" section of a data file, the line that corresponds to the 1st example above would be listed as

1 80.0 1 3 

See the dihedral_style command for more discussion of the formulas that use these coefficients. The units of each coefficient are shown in parenthesis.


For style charmm, specify 4 coefficients:

The weighting factor is applied to pairwise interaction between the 1st and 4th atoms in the dihedral.


For style class2, only coefficients for the Ed formula can be specified in the input script. These are the 6 coefficients:

Coefficients for all the other formulas must be specified in the data file.

For the Embt formula, the coefficients are listed under a "MiddleBondTorsion Coeffs" heading and each line lists 4 coefficients:

For the Eebt formula, the coefficients are listed under a "EndBondTorsion Coeffs" heading and each line lists 8 coefficients:

For the Eat formula, the coefficients are listed under a "AngleTorsion Coeffs" heading and each line lists 8 coefficients:

Theta1 and theta2 are specified in degrees, but LAMMPS converts them to radians internally; hence the units of D and E are in energy/radian.

For the Eaat formula, the coefficients are listed under a "AngleAngleTorsion Coeffs" heading and each line lists 3 coefficients:

Theta1 and theta2 are specified in degrees, but LAMMPS converts them to radians internally; hence the units of M are in energy/radian^2.

For the Ebb13 formula, the coefficients are listed under a "BondBond13 Coeffs" heading and each line lists 3 coefficients:


For style harmonic, specify 3 coefficients:


For style multiharmonic, specify 5 coefficients:

Restrictions:

This command must come after the simulation box is defined by a read_data, read_restart, or create_box command.

An dihedral style must be defined before any dihedral coefficients are set, either in the input script or in a data file.

Related commands:

dihedral_style

Default: none