forked from lijiext/lammps
163 lines
6.4 KiB
Plaintext
163 lines
6.4 KiB
Plaintext
.. index:: fix temp/rescale
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fix temp/rescale 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 temp/rescale N Tstart Tstop window fraction
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* temp/rescale = style name of this fix command
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* N = perform rescaling every N steps
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* Tstart,Tstop = desired temperature at start/end of run (temperature units)
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.. parsed-literal::
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Tstart can be a variable (see below)
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* window = only rescale if temperature is outside this window (temperature units)
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* fraction = rescale to target temperature by this fraction
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Examples
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""""""""
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.. parsed-literal::
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fix 3 flow temp/rescale 100 1.0 1.1 0.02 0.5
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fix 3 boundary temp/rescale 1 1.0 1.5 0.05 1.0
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fix 3 boundary temp/rescale 1 1.0 1.5 0.05 1.0
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Description
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"""""""""""
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Reset the temperature of a group of atoms by explicitly rescaling
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their velocities.
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The rescaling is applied to only the translational degrees of freedom
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for the particles, which is an important consideration if finite-size
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particles which have rotational degrees of freedom are being
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thermostatted with this fix. The translational degrees of freedom can
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also have a bias velocity removed from them before thermostatting
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takes place; see the description below.
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Rescaling is performed every N timesteps. The target temperature is a
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ramped value between the *Tstart* and *Tstop* temperatures at the
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beginning and end of the run.
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*Tstart* can be specified as an equal-style :doc:`variable <variable>`.
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In this case, the *Tstop* setting is ignored. If the value is a
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variable, it should be specified as v_name, where name is the variable
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name. In this case, the variable will be evaluated each timestep, and
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its value used to determine the target temperature.
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Equal-style variables can specify formulas with various mathematical
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functions, and include :doc:`thermo_style <thermo_style>` command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent temperature.
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Rescaling is only performed if the difference between the current and
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desired temperatures is greater than the *window* value. The amount
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of rescaling that is applied is a *fraction* (from 0.0 to 1.0) of the
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difference between the actual and desired temperature. E.g. if
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*fraction* = 1.0, the temperature is reset to exactly the desired
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value.
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.. note::
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Unlike the :doc:`fix nvt <fix_nh>` command which performs
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Nose/Hoover thermostatting AND time integration, this fix does NOT
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perform time integration. It only modifies velocities to effect
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thermostatting. Thus you must use a separate time integration fix,
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like :doc:`fix nve <fix_nve>` to actually update the positions of atoms
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using the modified velocities. Likewise, this fix should not normally
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be used on atoms that also have their temperature controlled by
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another fix - e.g. by :doc:`fix nvt <fix_nh>` or :doc:`fix langevin <fix_langevin>` commands.
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See :ref:`this howto section <howto_16>` of the manual for
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a discussion of different ways to compute temperature and perform
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thermostatting.
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This fix computes a temperature each timestep. To do this, the fix
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creates its own compute of style "temp", as if one of this command had
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been issued:
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.. parsed-literal::
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compute fix-ID_temp group-ID temp
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See the :doc:`compute temp <compute_temp>` for details. Note that the
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ID of the new compute is the fix-ID + underscore + "temp", and the
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group for the new compute is the same as the fix group.
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Note that this is NOT the compute used by thermodynamic output (see
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the :doc:`thermo_style <thermo_style>` command) with ID = *thermo_temp*\ .
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This means you can change the attributes of this fix's temperature
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(e.g. its degrees-of-freedom) via the
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:doc:`compute_modify <compute_modify>` command or print this temperature
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during thermodynamic output via the :doc:`thermo_style custom <thermo_style>` command using the appropriate compute-ID.
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It also means that changing attributes of *thermo_temp* will have no
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effect on this fix.
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Like other fixes that perform thermostatting, this fix can be used
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with :doc:`compute commands <compute>` that calculate a temperature
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after removing a "bias" from the atom velocities. E.g. removing the
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center-of-mass velocity from a group of atoms or only calculating
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temperature on the x-component of velocity or only calculating
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temperature for atoms in a geometric region. This is not done by
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default, but only if the :doc:`fix_modify <fix_modify>` command is used
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to assign a temperature compute to this fix that includes such a bias
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term. See the doc pages for individual :doc:`compute commands <compute>` to determine which ones include a bias. In
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this case, the thermostat works in the following manner: the current
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temperature is calculated taking the bias into account, bias is
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removed from each atom, thermostatting is performed on the remaining
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thermal degrees of freedom, and the bias is added back in.
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----------
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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>`.
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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 implied by a velocity rescaling 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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: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".
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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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Related commands
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""""""""""""""""
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:doc:`fix langevin <fix_langevin>`, :doc:`fix nvt <fix_nh>`,
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:doc:`fix_modify <fix_modify>`
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**Default:** none
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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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