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compute_pair.txt
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.. index:: compute pair
compute pair command
====================
Syntax
""""""
.. parsed-literal::
compute ID group-ID pair pstyle evalue
* ID, group-ID are documented in :doc:`compute <compute>` command
* pair = style name of this compute command
* pstyle = style name of a pair style that calculates additional values
* evalue = *epair* or *evdwl* or *ecoul* or blank (optional setting)
Examples
""""""""
.. parsed-literal::
compute 1 all pair gauss
compute 1 all pair lj/cut/coul/cut ecoul
compute 1 all pair reax
Description
"""""""""""
Define a computation that extracts additional values calculated by a
pair style, and makes them accessible for output or further processing
by other commands. The group specified for this command is ignored.
The specified *pstyle* must be a pair style used in your simulation
either by itself or as a sub-style in a :doc:`pair_style hybrid or hybrid/overlay <pair_hybrid>` command.
The *evalue* setting is optional; it may be left off the command. All
pair styles tally a potential energy *epair* which may be broken into
two parts: *evdwl* and *ecoul* such that *epair* = *evdwl* + *ecoul*\ .
If the pair style calculates Coulombic interactions, their energy will
be tallied in *ecoul*\ . Everything else (whether it is a Lennard-Jones
style van der Waals interaction or not) is tallied in *evdwl*\ . If
*evalue* is specified as *epair* or left out, then *epair* is stored
as a global scalar by this compute. This is useful when using
:doc:`pair_style hybrid <pair_hybrid>` if you want to know the portion
of the total energy contributed by one sub-style. If *evalue* is
specfied as *evdwl* or *ecoul*\ , then just that portion of the energy
is stored as a global scalar.
.. note::
The energy returned by the *evdwl* keyword does not include tail
corrections, even if they are enabled via the
:doc:`pair_modify <pair_modify>` command.
Some pair styles tally additional quantities, e.g. a breakdown of
potential energy into a dozen or so components is tallied by the
:doc:`pair_style reax <pair_reax>` commmand. These values (1 or more)
are stored as a global vector by this compute. See the doc page for
:doc:`individual pair styles <pair_style>` for info on these values.
**Output info:**
This compute calculates a global scalar which is *epair* or *evdwl* or
*ecoul*\ . If the pair style supports it, it also calculates a global
vector of length >= 1, as determined by the pair style. These values
can be used by any command that uses global scalar or vector values
from a compute as input. See :ref:`this section <howto_15>` for an overview of LAMMPS output
options.
The scalar and vector values calculated by this compute are
"extensive".
The scalar value will be in energy :doc:`units <units>`. The vector
values will typically also be in energy :doc:`units <units>`, but see
the doc page for the pair style for details.
Restrictions
""""""""""""
none
Related commands
""""""""""""""""
:doc:`compute pe <compute_pe>`, :doc:`compute bond <compute_bond>`
Default
"""""""
The default for *evalue* is *epair*\ .
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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