forked from lijiext/lammps
137 lines
4.3 KiB
Plaintext
137 lines
4.3 KiB
Plaintext
.. index:: fix langevin/eff
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fix langevin/eff command
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========================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID langevin/eff Tstart Tstop damp seed keyword values ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* langevin/eff = style name of this fix command
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* Tstart,Tstop = desired temperature at start/end of run (temperature units)
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* damp = damping parameter (time units)
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* seed = random number seed to use for white noise (positive integer)
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* zero or more keyword/value pairs may be appended
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.. parsed-literal::
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keyword = *scale* or *tally* or *zero*
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*scale* values = type ratio
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type = atom type (1-N)
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ratio = factor by which to scale the damping coefficient
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*tally* values = *no* or *yes*
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*no* = do not tally the energy added/subtracted to atoms
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*yes* = do tally the energy added/subtracted to atoms
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.. parsed-literal::
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*zero* value = *no* or *yes*
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*no* = do not set total random force to zero
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*yes* = set total random force to zero
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Examples
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""""""""
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.. parsed-literal::
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fix 3 boundary langevin/eff 1.0 1.0 10.0 699483
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fix 1 all langevin/eff 1.0 1.1 10.0 48279 scale 3 1.5
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Description
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"""""""""""
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Apply a Langevin thermostat as described in :ref:`(Schneider) <Schneider>`
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to a group of nuclei and electrons in the :doc:`electron force field <pair_eff>` model. Used with :doc:`fix nve/eff <fix_nve_eff>`,
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this command performs Brownian dynamics (BD), since the total force on
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each atom will have the form:
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.. parsed-literal::
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F = Fc + Ff + Fr
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Ff = - (m / damp) v
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Fr is proportional to sqrt(Kb T m / (dt damp))
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Fc is the conservative force computed via the usual inter-particle
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interactions (:doc:`pair_style <pair_style>`).
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The Ff and Fr terms are added by this fix on a per-particle basis.
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The operation of this fix is exactly like that described by the :doc:`fix langevin <fix_langevin>` command, except that the thermostatting
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is also applied to the radial electron velocity for electron
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particles.
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Restart, fix_modify, output, run start/stop, minimize info
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""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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No information about this fix is written to :doc:`binary restart files <restart>`. Because the state of the random number generator
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is not saved in restart files, this means you cannot do "exact"
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restarts with this fix, where the simulation continues on the same as
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if no restart had taken place. However, in a statistical sense, a
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restarted simulation should produce the same behavior.
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The :doc:`fix_modify <fix_modify>` *temp* option is supported by this
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fix. You can use it to assign a temperature :doc:`compute <compute>`
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you have defined to this fix which will be used in its thermostatting
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procedure, as described above. For consistency, the group used by
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this fix and by the compute should be the same.
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The :doc:`fix_modify <fix_modify>` *energy* option is supported by this
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fix to add the energy change induced by Langevin thermostatting to the
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system's potential energy as part of :doc:`thermodynamic output <thermo_style>`. Note that use of this option requires
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setting the *tally* keyword to *yes*\ .
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This fix computes a global scalar which can be accessed by various
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:ref:`output commands <howto_15>`. The scalar is the
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cummulative energy change due to this fix. The scalar value
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calculated by this fix is "extensive". Note that calculation of this
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quantity requires setting the *tally* keyword to *yes*\ .
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This fix can ramp its target temperature over multiple runs, using the
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*start* and *stop* keywords of the :doc:`run <run>` command. See the
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:doc:`run <run>` command for details of how to do this.
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This fix is not invoked during :doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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none
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This fix is part of the USER-EFF package. It is only enabled if
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LAMMPS was built with that package. See the :ref:`Making LAMMPS <start_3>` section for more info.
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Related commands
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""""""""""""""""
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:doc:`fix langevin <fix_langevin>`
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Default
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"""""""
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The option defaults are scale = 1.0 for all types and tally = no.
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----------
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.. _Dunweg:
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**(Dunweg)** Dunweg and Paul, Int J of Modern Physics C, 2, 817-27 (1991).
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.. _Schneider:
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**(Schneider)** Schneider and Stoll, Phys Rev B, 17, 1302 (1978).
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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