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### improper_coeff command

Syntax:

    improper_coeff N args

- N = improper type (see asterisk form below)
- args = coefficients for one or more improper types

Examples:

    improper_coeff 1 300.0 0.0
    improper_coeff * 80.2 -1 2
    improper_coeff *4 80.2 -1 2

Description:

Specify the improper force field coefficients for one or more improper types. The number and meaning of the coefficients depends on the improper style. Improper 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 asterisk can be used to set the coefficients for multiple improper types. This takes the form "*" or "*n" or "n*" or "m*n". If N = the number of improper types, then an asterisk with no numeric values means all types from 1 to N. A leading asterisk means all types from 1 to n (inclusive). A trailing asterisk means all types from n to N (inclusive). A middle asterisk means all types from m to n (inclusive).

Note that using an improper_coeff command can override a previous setting for the same improper type. For example, these commands set the coeffs for all improper types, then overwrite the coeffs for just improper type 2:

    improper_coeff * 300.0 0.0
    improper_coeff 2 50.0 0.0

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

    1 300.0 0.0

The improper_style class2 is an exception to this rule, in that an additional argument is used in the input script to allow specification of the cross-term coefficients. See its doc page for details.

Here is an alphabetic list of improper styles defined in LAMMPS. Click on the style to display the formula it computes and coefficients specified by the associated improper_coeff command:

- improper_style none - turn off improper interactions
- improper_style hybrid - define multiple styles of improper interactions

- improper_style class2 - COMPASS (class 2) improper
- improper_style cvff - CVFF improper
- improper_style harmonic - harmonic improper
- improper_style umbrella - DREIDING improper

There are also additional improper styles submitted by users which are included in the LAMMPS distribution. The list of these with links to the individual styles are given in the improper section of this page.

Restrictions:

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

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

Related commands:

improper_style

Default: none