# improper_style hybrid command¶

## Syntax¶

improper_style hybrid style1 style2 ...

• style1,style2 = list of one or more improper styles

## Examples¶

improper_style hybrid harmonic helix
improper_coeff 1 harmonic 120.0 30
improper_coeff 2 cvff 20.0 -1 2


## Description¶

The hybrid style enables the use of multiple improper styles in one simulation. An improper style is assigned to each improper type. For example, impropers in a polymer flow (of improper type 1) could be computed with a harmonic potential and impropers in the wall boundary (of improper type 2) could be computed with a cvff potential. The assignment of improper type to style is made via the improper_coeff command or in the data file.

In the improper_coeff command, the first coefficient sets the improper style and the remaining coefficients are those appropriate to that style. In the example above, the 2 improper_coeff commands would set impropers of improper type 1 to be computed with a harmonic potential with coefficients 120.0, 30 for $$K$$, $$\chi_0$$. Improper type 2 would be computed with a cvff potential with coefficients 20.0, -1, 2 for K, d, and n, respectively.

If improper coefficients are specified in the data file read via the read_data command, then the same rule applies. E.g. “harmonic” or “cvff”, must be added after the improper type, for each line in the “Improper Coeffs” section, e.g.

Improper Coeffs

1 harmonic 120.0 30
2 cvff 20.0 -1 2
...


If class2 is one of the improper hybrid styles, the same rule holds for specifying additional AngleAngle coefficients either via the input script or in the data file. I.e. class2 must be added to each line after the improper type. For lines in the AngleAngle Coeffs section of the data file for dihedral types that are not class2, you must use an improper style of skip as a placeholder, e.g.

AngleAngle Coeffs

1 skip
2 class2 0.0 0.0 0.0 115.06 130.01 115.06
...


Note that it is not necessary to use the improper style skip in the input script, since AngleAngle coefficients need not be specified at all for improper types that are not class2.

An improper style of none can be specified as the second argument to the improper_coeff command, if you desire to turn off certain improper types.

## Restrictions¶

This improper style can only be used if LAMMPS was built with the MOLECULE package. See the Build package doc page for more info.

Unlike other improper styles, the hybrid improper style does not store improper coefficient info for individual sub-styles in a binary restart files. Thus when restarting a simulation from a restart file, you need to re-specify improper_coeff commands.

none