92 lines
2.7 KiB
ReStructuredText
92 lines
2.7 KiB
ReStructuredText
.. index:: compute pe
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compute pe command
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==================
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Syntax
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""""""
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.. code-block:: LAMMPS
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compute ID group-ID pe keyword ...
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* pe = style name of this compute command
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* zero or more keywords may be appended
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* keyword = *pair* or *bond* or *angle* or *dihedral* or *improper* or *kspace* or *fix*
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Examples
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""""""""
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.. code-block:: LAMMPS
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compute 1 all pe
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compute molPE all pe bond angle dihedral improper
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Description
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"""""""""""
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Define a computation that calculates the potential energy of the
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entire system of atoms. The specified group must be "all". See the
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:doc:`compute pe/atom <compute_pe_atom>` command if you want per-atom
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energies. These per-atom values could be summed for a group of atoms
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via the :doc:`compute reduce <compute_reduce>` command.
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The energy is calculated by the various pair, bond, etc. potentials
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defined for the simulation. If no extra keywords are listed, then the
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potential energy is the sum of pair, bond, angle, dihedral, improper,
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:math:`k`-space (long-range), and fix energy (i.e., it is as though all the
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keywords were listed). If any extra keywords are listed, then only
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those components are summed to compute the potential energy.
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The :math:`k`-space contribution requires 1 extra FFT each timestep the energy
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is calculated, if using the PPPM solver via the :doc:`kspace_style pppm <kspace_style>` command. Thus it can increase the cost of the
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PPPM calculation if it is needed on a large fraction of the simulation
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timesteps.
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Various fixes can contribute to the total potential energy of the
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system if the *fix* contribution is included. See the doc pages for
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:doc:`individual fixes <fix>` for details of which ones compute a
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potential energy.
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.. note::
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The :doc:`fix_modify energy yes <fix_modify>` command must also be
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specified if a fix is to contribute potential energy to this command.
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A compute of this style with the ID of "thermo_pe" is created when
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LAMMPS starts up, as if this command were in the input script:
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.. code-block:: LAMMPS
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compute thermo_pe all pe
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See the "thermo_style" command for more details.
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----------
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Output info
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"""""""""""
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This compute calculates a global scalar (the potential energy). This
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value can be used by any command that uses a global scalar value from
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a compute as input. See the :doc:`Howto output <Howto_output>` doc page
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for an overview of LAMMPS output options.
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The scalar value calculated by this compute is "extensive". The
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scalar value will be in energy :doc:`units <units>`.
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Restrictions
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""""""""""""
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none
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Related commands
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""""""""""""""""
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:doc:`compute pe/atom <compute_pe_atom>`
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Default
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"""""""
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none
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