113 lines
3.8 KiB
ReStructuredText
113 lines
3.8 KiB
ReStructuredText
.. index:: compute temp/eff
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compute temp/eff 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 temp/eff
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* temp/eff = style name of this compute command
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Examples
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""""""""
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.. code-block:: LAMMPS
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compute 1 all temp/eff
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compute myTemp mobile temp/eff
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Description
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"""""""""""
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Define a computation that calculates the temperature of a group of
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nuclei and electrons in the :doc:`electron force field <pair_eff>`
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model. A compute of this style can be used by commands that compute a
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temperature (e.g., :doc:`thermo_modify <thermo_modify>`,
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:doc:`fix npt/eff <fix_nh_eff>`).
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The temperature is calculated by the formula
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.. math::
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\text{KE} = \frac{\text{dim}}{2} N k_B T,
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where KE is the total kinetic energy of the group of atoms (sum of
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:math:`\frac12 m v^2` for nuclei and sum of
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:math:`\frac12 (m v^2 + \frac34 m s^2`) for electrons, where :math:`s`
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includes the radial electron velocity contributions), dim = 2 or 3 is the
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dimensionality of the simulation, :math:`N` is the number of atoms (only total
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number of nuclei in the eFF (see the :doc:`pair_eff <pair_style>`
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command) in the group, :math:`k_B` is the Boltzmann constant, and :math:`T` is
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the absolute temperature. This expression is summed over all nuclear and
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electronic degrees of freedom, essentially by setting the kinetic contribution
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to the heat capacity to :math:`\frac32 k` (where only nuclei contribute). This
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subtlety is valid for temperatures well below the Fermi temperature, which for
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densities two to five times the density of liquid hydrogen ranges from
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86,000 to 170,000 K.
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.. note::
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For eFF models, in order to override the default temperature
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reported by LAMMPS in the thermodynamic quantities reported via the
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:doc:`thermo <thermo>` command, the user should apply a
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:doc:`thermo_modify <thermo_modify>` command, as shown in the following
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example:
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.. code-block:: LAMMPS
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compute effTemp all temp/eff
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thermo_style custom step etotal pe ke temp press
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thermo_modify temp effTemp
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A six-component kinetic energy tensor is also calculated by this compute
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for use in the computation of a pressure tensor. The formula for the
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components of the tensor is the same as the above formula, except that
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:math:`v^2` is replaced by :math:`v_x v_y` for the :math:`xy` component, etc.
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For the eFF, again, the radial electronic velocities are also considered.
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The number of atoms contributing to the temperature is assumed to be
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constant for the duration of the run; use the *dynamic* option of the
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:doc:`compute_modify <compute_modify>` command if this is not the case.
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This compute subtracts out degrees-of-freedom due to fixes that
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constrain molecular motion, such as :doc:`fix shake <fix_shake>` and
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:doc:`fix rigid <fix_rigid>`. This means the temperature of groups of
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atoms that include these constraints will be computed correctly. If
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needed, the subtracted degrees-of-freedom can be altered using the
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*extra* option of the :doc:`compute_modify <compute_modify>` command.
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See the :doc:`Howto thermostat <Howto_thermostat>` page for a
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discussion of different ways to compute temperature and perform
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thermostatting.
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Output info
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"""""""""""
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The scalar value calculated by this compute is "intensive", meaning it
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is independent of the number of atoms in the simulation. The vector
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values are "extensive", meaning they scale with the number of atoms in
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the simulation.
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Restrictions
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""""""""""""
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This compute is part of the EFF package. It is only enabled if
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LAMMPS was built with that package. See the :doc:`Build package <Build_package>` page for more info.
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Related commands
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""""""""""""""""
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:doc:`compute temp/partial <compute_temp_partial>`,
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:doc:`compute temp/region <compute_temp_region>`,
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:doc:`compute pressure <compute_pressure>`
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Default
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"""""""
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none
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