.. index:: compute pe/atom

compute pe/atom command
=======================

Syntax
""""""

.. code-block:: LAMMPS

   compute ID group-ID pe/atom keyword ...

* ID, group-ID are documented in :doc:`compute <compute>` command
* pe/atom = style name of this compute command
* zero or more keywords may be appended
* keyword = *pair* or *bond* or *angle* or *dihedral* or *improper* or *kspace* or *fix*

Examples
""""""""

.. code-block:: LAMMPS

   compute 1 all pe/atom
   compute 1 all pe/atom pair
   compute 1 all pe/atom pair bond

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

Define a computation that computes the per-atom potential energy for
each atom in a group.  See the :doc:`compute pe <compute_pe>` command if
you want the potential energy of the entire system.

The per-atom energy is calculated by the various pair, bond, etc
potentials defined for the simulation.  If no extra keywords are
listed, then the potential energy is the sum of pair, bond, angle,
dihedral, improper, :math:`k`-space (long-range), and fix energy (i.e., it is as
though all the keywords were listed).  If any extra keywords are listed,
then only those components are summed to compute the potential energy.

Note that the energy of each atom is due to its interaction with all
other atoms in the simulation, not just with other atoms in the group.

For an energy contribution produced by a small set of atoms (e.g., 4
atoms in a dihedral or 3 atoms in a Tersoff 3-body interaction), that
energy is assigned in equal portions to each atom in the set (e.g., 1/4 of the
dihedral energy to each of the four atoms).

The :doc:`dihedral_style charmm <dihedral_charmm>` style calculates
pairwise interactions between 1--4 atoms.  The energy contribution of
these terms is included in the pair energy, not the dihedral energy.

The KSpace contribution is calculated using the method in
:ref:`(Heyes) <Heyes1>` for the Ewald method and a related method for PPPM,
as specified by the :doc:`kspace_style pppm <kspace_style>` command.
For PPPM, the calculation requires 1 extra FFT each timestep that
per-atom energy is calculated.  This `document <PDF/kspace.pdf>`_
describes how the long-range per-atom energy calculation is performed.

Various fixes can contribute to the per-atom potential energy of the
system if the *fix* contribution is included.  See the doc pages for
:doc:`individual fixes <fix>` for details of which ones compute a
per-atom potential energy.

.. note::

   The :doc:`fix_modify energy yes <fix_modify>` command must also be
   specified if a fix is to contribute per-atom potential energy to this
   command.

As an example of per-atom potential energy compared to total potential
energy, these lines in an input script should yield the same result
in the last 2 columns of thermo output:

.. code-block:: LAMMPS

   compute        peratom all pe/atom
   compute        pe all reduce sum c_peratom
   thermo_style   custom step temp etotal press pe c_pe

.. note::

   The per-atom energy does not include any Lennard-Jones tail
   corrections to the energy added by the
   :doc:`pair_modify tail yes <pair_modify>` command, since those are
   contributions to the global system energy.

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

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

The per-atom vector values will be in energy :doc:`units <units>`.

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

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

:doc:`compute pe <compute_pe>`, :doc:`compute stress/atom <compute_stress_atom>`

Default
"""""""

none

----------

.. _Heyes1:

**(Heyes)** Heyes, Phys Rev B 49, 755 (1994),
