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### dihedral_style charmm command

Syntax:

    dihedral_style charmm

Examples:

    dihedral_style charmm
    dihedral_coeff 1 120.0 1 60 0.5

Description:

The charmm dihedral style uses the potential

See (MacKerell) for a description of the CHARMM force field. This dihedral style can also be used for the AMBER force field (see comment on weighting factors below). See (Cornell) for a description of the AMBER force field.

The following coefficients must be defined for each dihedral type via the dihedral_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)
- n (integer >= 0)
- d (integer value of degrees)
- weighting factor (0.0 to 1.0)

The weighting factor is applied to pairwise interaction between the 1st and 4th atoms in the dihedral, which are computed by a CHARMM pair_style with epsilon and sigma values specified with a pair_coeff command. Note that this weighting factor is unrelated to the weighting factor specified by the special bonds command which applies to all 1-4 interactions in the system.

For CHARMM force fields, the special_bonds 1-4 weighting factor should be set to 0.0. This is because the pair styles that contain "charmm" (e.g. pair_style lj/charmm/coul/long) define extra 1-4 interaction coefficients that are used by this dihedral style to compute those interactions explicitly. This means that if any of the weighting factors defined as dihedral coefficients (4th coeff above) are non-zero, then you must use a charmm pair style. Note that if you do not set the special_bonds 1-4 weighting factor to 0.0 (which is the default) then 1-4 interactions in dihedrals will be computed twice, once by the pair routine and once by the dihedral routine, which is probably not what you want.

For AMBER force fields, the special_bonds 1-4 weighting factor should be set to the AMBER defaults (1/2 and 5/6) and all the dihedral weighting factors (4th coeff above) should be set to 0.0. In this case, you can use any pair style you wish, since the dihedral does not need any 1-4 information.

Restrictions:

This dihedral style can only be used if LAMMPS was built with the "molecular" package (which it is by default). See the Making LAMMPS section for more info on packages.

Related commands:

dihedral_coeff

Default: none

(Cornell) Cornell, Cieplak, Bayly, Gould, Merz, Ferguson, Spellmeyer, Fox, Caldwell, Kollman, JACS 117, 5179-5197 (1995).

(MacKerell) MacKerell, Bashford, Bellott, Dunbrack, Evanseck, Field, Fischer, Gao, Guo, Ha, et al, J Phys Chem B, 102, 3586 (1998).