.. index:: compute dihedral/local

compute dihedral/local command
==============================

Syntax
""""""

.. code-block:: LAMMPS

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

* ID, group-ID are documented in :doc:`compute <compute>` command
* dihedral/local = style name of this compute command
* one or more values may be appended
* value = *phi* or *v_name*

  .. parsed-literal::

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

* zero or more keyword/args pairs may be appended
* keyword = *set*

  .. parsed-literal::

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

Examples
""""""""

.. code-block:: 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* (:math:`\phi`) is the dihedral angle, as defined in
the diagram on the :doc:`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 :math:`\phi`.
The *name* specified for the *v_name* value is the name of an
:doc:`equal-style variable <variable>` which should evaluate a formula
based on a variable which will store the angle :math:`\phi`.  This
other variable must be an :doc:`internal-style variable <variable>`
specified by the *set* keyword.  It is an internal-style variable,
because this command resets its value directly.  The internal-style
variable does not need to be defined in the input script (though it
can be); if it is not defined, then the *set* option creates an
:doc:`internal-style variable <variable>` with the specified name.

Note that the value of :math:`\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 :math:`\cos\phi` and :math:`\cos^2\phi` of every
dihedral angle in the system and output the statistics in various
ways:

.. code-block:: LAMMPS

   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[*] inputs local
   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 :doc:`dump local <dump>` command will output the angle (:math:`\phi`),
:math:`\cos(\phi)`, and :math:`\cos^2(\phi)` for every dihedral in the system.
The :doc:`thermo_style <thermo_style>` command will print the average of
those quantities via the :doc:`compute reduce <compute_reduce>` command
with thermo output.  And the :doc:`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 :doc:`compute property/local
<compute_property_local>` command can be combined with data from this
command and output by the :doc:`dump local <dump>` command in a
consistent way.

Here is an example of how to do this:

.. code-block:: 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
:doc:`Howto output <Howto_output>` page for an overview of LAMMPS output
options.

The output for *phi* will be in degrees.

Restrictions
""""""""""""
 none

Related commands
""""""""""""""""

:doc:`dump local <dump>`, :doc:`compute property/local <compute_property_local>`

Default
"""""""

none
