.. index:: compute ave/sphere/atom
.. index:: compute ave/sphere/atom/kk

compute ave/sphere/atom command
================================

Accelerator Variants: *ave/sphere/atom/kk*

Syntax
""""""

.. code-block:: LAMMPS

   compute ID group-ID ave/sphere/atom keyword values ...

* ID, group-ID are documented in :doc:`compute <compute>` command
* ave/sphere/atom = style name of this compute command
* one or more keyword/value pairs may be appended

  .. parsed-literal::

     keyword = *cutoff*
       *cutoff* value = distance cutoff

Examples
""""""""

.. code-block:: LAMMPS

   compute 1 all ave/sphere/atom

   compute 1 all ave/sphere/atom cutoff 5.0
   comm_modify cutoff 5.0

Description
"""""""""""

.. versionadded:: 7Jan2022

Define a computation that calculates the local mass density and
temperature for each atom based on its neighbors inside a spherical
cutoff.  If an atom has :math:`M` neighbors, then its local mass density is
calculated as the sum of its mass and its :math:`M` neighbor masses, divided
by the volume of the cutoff sphere (or circle in 2d).  The local
temperature of the atom is calculated as the temperature of the
collection of :math:`M+1` atoms, after subtracting the center-of-mass velocity
of the :math:`M+1` atoms from each of the :math:`M+1` atom's velocities.  This
is effectively the thermal velocity of the neighborhood of the central
atom, similar to :doc:`compute temp/com <compute_temp_com>`.

The optional keyword *cutoff* defines the distance cutoff used when
searching for neighbors. The default value is the cutoff specified by
the pair style. If no pair style is defined, then a cutoff must be
defined using this keyword. If the specified cutoff is larger than
that of the pair_style plus neighbor skin (or no pair style is
defined), the *comm_modify cutoff* option must also be set to match
that of the *cutoff* keyword.

The neighbor list needed to compute this quantity is constructed each
time the calculation is performed (i.e. each time a snapshot of atoms
is dumped).  Thus it can be inefficient to compute/dump this quantity
too frequently.

.. note::

   If you have a bonded system, then the settings of
   :doc:`special_bonds <special_bonds>` command can remove pairwise
   interactions between atoms in the same bond, angle, or dihedral.
   This is the default setting for the :doc:`special_bonds
   <special_bonds>` command, and means those pairwise interactions do
   not appear in the neighbor list.  Because this compute uses the
   neighbor list, it also means those pairs will not be included in
   the order parameter.  This difficulty can be circumvented by
   writing a dump file, and using the :doc:`rerun <rerun>` command to
   compute the order parameter for snapshots in the dump file.  The
   rerun script can use a :doc:`special_bonds <special_bonds>` command
   that includes all pairs in the neighbor list.

----------


.. include:: accel_styles.rst


----------

Output info
"""""""""""

This compute calculates a per-atom array with two columns: mass
density in density :doc:`units <units>` and temperature in temperature
:doc:`units <units>`.

These values can be accessed by any command that uses per-atom values
from a compute as input.  See the :doc:`Howto output <Howto_output>`
doc page for an overview of LAMMPS output options.

Restrictions
""""""""""""

This compute is part of the EXTRA-COMPUTE package.  It is only enabled
if LAMMPS was built with that package.  See the :doc:`Build package
<Build_package>` page for more info.

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

:doc:`comm_modify <comm_modify>`

Default
"""""""

The option defaults are *cutoff* = pair style cutoff.

