# compute dihedral/local command

## Syntax

``` LAMMPS
compute ID group-ID dihedral/local value1 value2 ... keyword args ...
```

-   ID, group-ID are documented in [compute](compute) command

-   dihedral/local = style name of this compute command

-   one or more values may be appended

-   value = *phi* or *v_name*

        *phi* = tabulate dihedral angles
        *v_name* = equal-style variable with name (see below)

-   zero or more keyword/args pairs may be appended

-   keyword = *set*

        *set* args = phi name
          phi = only currently allowed arg
          name = name of variable to set with phi

## Examples

``` LAMMPS
compute 1 all dihedral/local phi
compute 1 all dihedral/local phi v_cos set phi p
```

## Description

Define a computation that calculates properties of individual dihedral
interactions. The number of datums generated, aggregated across all
processors, equals the number of dihedral angles in the system, modified
by the group parameter as explained below.

The value *phi* ($\phi$) is the dihedral angle, as defined in the
diagram on the [dihedral_style](dihedral_style) doc page.

The value *v_name* can be used together with the *set* keyword to
compute a user-specified function of the dihedral angle $\phi$. The
*name* specified for the *v_name* value is the name of an [equal-style
variable](variable) which should evaluate a formula based on a variable
which will store the angle $\phi$. This other variable must be an
[internal-style variable](variable) defined in the input script; its
initial numeric value can be anything. It must be an internal-style
variable, because this command resets its value directly. The *set*
keyword is used to identify the name of this other variable associated
with $\phi$.

Note that the value of $\phi$ for each angle which stored in the
internal variable is in radians, not degrees.

As an example, these commands can be added to the bench/in.rhodo script
to compute the $\cos\phi$ and $\cos^2\phi$ of every dihedral angle in
the system and output the statistics in various ways:

``` LAMMPS
variable p internal 0.0
variable cos equal cos(v_p)
variable cossq equal cos(v_p)*cos(v_p)

compute 1 all property/local datom1 datom2 datom3 datom4 dtype
compute 2 all dihedral/local phi v_cos v_cossq set phi p
dump 1 all local 100 tmp.dump c_1[*] c_2[*]

compute 3 all reduce ave c_2[*]
thermo_style custom step temp press c_3[*]

fix 10 all ave/histo 10 10 100 -1 1 20 c_2[2] mode vector file tmp.histo
```

The [dump local](dump) command will output the angle ($\phi$),
$\cos(\phi)$, and $\cos^2(\phi)$ for every dihedral in the system. The
[thermo_style](thermo_style) command will print the average of those
quantities via the [compute reduce](compute_reduce) command with thermo
output. And the [fix ave/histo](fix_ave_histo) command will histogram
the cosine(angle) values and write them to a file.

------------------------------------------------------------------------

The local data stored by this command is generated by looping over all
the atoms owned on a processor and their dihedrals. A dihedral will only
be included if all four atoms in the dihedral are in the specified
compute group.

Note that as atoms migrate from processor to processor, there will be no
consistent ordering of the entries within the local vector or array from
one timestep to the next. The only consistency that is guaranteed is
that the ordering on a particular timestep will be the same for local
vectors or arrays generated by other compute commands. For example,
dihedral output from the [compute
property/local](compute_property_local) command can be combined with
data from this command and output by the [dump local](dump) command in a
consistent way.

Here is an example of how to do this:

``` LAMMPS
compute 1 all property/local dtype datom1 datom2 datom3 datom4
compute 2 all dihedral/local phi
dump 1 all local 1000 tmp.dump index c_1[1] c_1[2] c_1[3] c_1[4] c_1[5] c_2[1]
```

## Output info

This compute calculates a local vector or local array depending on the
number of values. The length of the vector or number of rows in the
array is the number of dihedrals. If a single value is specified, a
local vector is produced. If two or more values are specified, a local
array is produced where the number of columns is equal to the number of
values. The vector or array can be accessed by any command that uses
local values from a compute as input. See the [Howto
output](Howto_output) page for an overview of LAMMPS output options.

The output for *phi* will be in degrees.

## Restrictions

> none

## Related commands

[dump local](dump), [compute property/local](compute_property_local)

## Default

none