126 lines
4.7 KiB
ReStructuredText
126 lines
4.7 KiB
ReStructuredText
.. index:: fix ffl
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fix ffl 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 id-group ffl tau Tstart Tstop seed [flip-type]
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* ffl = style name of this fix command
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* tau = thermostat parameter (positive real)
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* Tstart, Tstop = temperature ramp during the run
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* seed = random number seed to use for generating noise (positive integer)
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* one more value may be appended
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.. parsed-literal::
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flip-type = determines the flipping type, can be chosen between rescale - no_flip - hard - soft, if no flip type is given, rescale will be chosen by default
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix 3 boundary ffl 10 300 300 31415
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fix 1 all ffl 100 500 500 9265 soft
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Description
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"""""""""""
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Apply a Fast-Forward Langevin Equation (FFL) thermostat as described
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in :ref:`(Hijazi) <Hijazi>`. Contrary to
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:doc:`fix langevin <fix_langevin>`, this fix performs both
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thermostatting and evolution of the Hamiltonian equations of motion, so it
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should not be used together with :doc:`fix nve <fix_nve>` -- at least not
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on the same atom groups.
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The time-evolution of a single particle undergoing Langevin dynamics is described
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by the equations
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.. math::
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\frac {dq}{dt} = \frac{p}{m},
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.. math::
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\frac {dp}{dt} = -\gamma p + W + F,
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where :math:`F` is the physical force, :math:`\gamma` is the friction coefficient, and :math:`W` is a
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Gaussian random force.
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The friction coefficient is the inverse of the thermostat parameter : :math:`\gamma = 1/\tau`, with :math:`\tau` the thermostat parameter *tau*\ .
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The thermostat parameter is given in the time units, :math:`\gamma` is in inverse time units.
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Equilibrium sampling a temperature T is obtained by specifying the
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target value as the *Tstart* and *Tstop* arguments, so that the internal
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constants depending on the temperature are computed automatically.
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The random number *seed* must be a positive integer. A Marsaglia random
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number generator is used. Each processor uses the input seed to
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generate its own unique seed and its own stream of random numbers.
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Thus the dynamics of the system will not be identical on two runs on
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different numbers of processors.
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The flipping type *flip-type* can be chosen between 4 types described in
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:ref:`(Hijazi) <Hijazi>`. The flipping operation occurs during the thermostatting
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step and it flips the momenta of the atoms. If no_flip is chosen, no flip
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will be executed and the integration will be the same as a standard
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Langevin thermostat :ref:`(Bussi) <Bussi3>`. The other flipping types are : rescale - hard - soft.
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**Restart, fix_modify, output, run start/stop, minimize info:**
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The instantaneous values of the extended variables are written to
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:doc:`binary restart files <restart>`. Because the state of the random
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number generator is not saved in restart files, this means you cannot
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do "exact" restarts with this fix, where the simulation continues on
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the same as if no restart had taken place. However, in a statistical
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sense, a restarted simulation should produce the same behavior.
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Note however that you should use a different seed each time you
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restart, otherwise the same sequence of random numbers will be used
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each time, which might lead to stochastic synchronization and
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subtle artifacts in the sampling.
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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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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>`.
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This fix computes a global scalar which can be accessed by various
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:doc:`output commands <Howto_output>`. The scalar is the cumulative
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energy change due to this fix. The scalar value calculated by this
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fix is "extensive".
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Restrictions
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""""""""""""
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In order to perform constant-pressure simulations please use
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:doc:`fix press/berendsen <fix_press_berendsen>`, rather than
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:doc:`fix npt <fix_nh>`, to avoid duplicate integration of the
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equations of motion.
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This fix is part of the USER-MISC package. It is only enabled if
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LAMMPS was built with that package. See the :doc:`Build package <Build_package>` doc page for more info.
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Related commands
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""""""""""""""""
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:doc:`fix nvt <fix_nh>`, :doc:`fix temp/rescale <fix_temp_rescale>`, :doc:`fix viscous <fix_viscous>`, :doc:`fix nvt <fix_nh>`, :doc:`pair_style dpd/tstat <pair_dpd>`, :doc:`fix gld <fix_gld>`, :doc:`fix gle <fix_gle>`
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----------
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.. _Hijazi:
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.. _Bussi3:
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**(Hijazi)** M. Hijazi, D. M. Wilkins, M. Ceriotti, J. Chem. Phys. 148, 184109 (2018)
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**(Bussi)** G. Bussi, M. Parrinello, Phs. Rev. E 75, 056707 (2007)
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