39ea6ef30e
use the (newly redefined) \AA macro for Angstrom symbol throughout
instead of either \mathring{A} or AA
128 lines
4.9 KiB
ReStructuredText
128 lines
4.9 KiB
ReStructuredText
.. index:: compute temp/ramp
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compute temp/ramp 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/ramp vdim vlo vhi dim clo chi keyword value ...
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* temp/ramp = style name of this compute command
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* vdim = *vx* or *vy* or *vz*
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* vlo,vhi = subtract velocities between vlo and vhi (velocity units)
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* dim = *x* or *y* or *z*
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* clo,chi = lower and upper bound of domain to subtract from (distance units)
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* zero or more keyword/value pairs may be appended
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* keyword = *units*
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.. parsed-literal::
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*units* value = *lattice* or *box*
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Examples
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""""""""
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.. code-block:: LAMMPS
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compute 2nd middle temp/ramp vx 0 8 y 2 12 units lattice
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Description
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"""""""""""
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Define a computation that calculates the temperature of a group of atoms,
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after subtracting out an ramped velocity profile before computing the kinetic
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energy. A compute of this style can be used by any command that computes a
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temperature (e.g. :doc:`thermo_modify <thermo_modify>`,
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:doc:`fix temp/rescale <fix_temp_rescale>`, :doc:`fix npt <fix_nh>`).
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The meaning of the arguments for this command which define the
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velocity ramp are the same as for the :doc:`velocity ramp <velocity>`
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command which was presumably used to impose the velocity.
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After the ramp velocity has been subtracted from the specified
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dimension for each atom, 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`), dim = 2 or 3 is the dimensionality of the simulation,
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:math:`N` is the number of atoms in the group, :math:`k_B` is the Boltzmann
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constant, and :math:`T` is the absolute temperature.
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The *units* keyword determines the meaning of the distance units used
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for coordinates (*clo*, *chi*) and velocities (*vlo*, *vhi*). A *box* value
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selects standard distance units as defined by the :doc:`units <units>`
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command (e.g., :math:`\AA` for units = real or metal). A
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*lattice* value means the distance units are in lattice spacings (i.e.,
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velocity in lattice spacings per unit time). The :doc:`lattice <lattice>`
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command must have been previously used to define the lattice spacing.
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A kinetic energy tensor, stored as a six-element vector, is also calculated by
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this compute for use in the computation of a pressure tensor. The formula for
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the 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, and
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so on. The six components of the vector are ordered :math:`xx`, :math:`yy`,
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:math:`zz`, :math:`xy`, :math:`xz`, :math:`yz`.
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The number of atoms contributing to the temperature is assumed to be constant
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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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The removal of the ramped velocity component by this fix is
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essentially computing the temperature after a "bias" has been removed
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from the velocity of the atoms. If this compute is used with a fix
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command that performs thermostatting then this bias will be subtracted
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from each atom, thermostatting of the remaining thermal velocity will
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be performed, and the bias will be added back in. Thermostatting
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fixes that work in this way include :doc:`fix nvt <fix_nh>`,
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:doc:`fix temp/rescale <fix_temp_rescale>`,
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:doc:`fix temp/berendsen <fix_temp_berendsen>`, and
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:doc:`fix langevin <fix_langevin>`.
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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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This compute calculates a global scalar (the temperature) and a global
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vector of length 6 (KE tensor), which can be accessed by indices 1--6.
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These values can be used by any command that uses global scalar or
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vector values from a compute as input. See the
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:doc:`Howto output <Howto_output>` page for an overview of LAMMPS output
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options.
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The scalar value calculated by this compute is "intensive". The
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vector values are "extensive".
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The scalar value will be in temperature :doc:`units <units>`. The
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vector values 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 temp <compute_temp>`, :doc:`compute temp/profie <compute_temp_profile>`, :doc:`compute temp/deform <compute_temp_deform>`, :doc:`compute pressure <compute_pressure>`
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Default
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"""""""
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The option default is units = lattice.
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