# improper_style hybrid command

## Syntax

``` LAMMPS
improper_style hybrid style1 style2 ...
```

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

## Examples

``` LAMMPS
improper_style hybrid harmonic cvff
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](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](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](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](restart). Thus when restarting a simulation from a restart file,
you need to re-specify improper_coeff commands.

## Related commands

[improper_coeff](improper_coeff)

## Default

none