forked from lijiext/lammps
106 lines
3.6 KiB
ReStructuredText
106 lines
3.6 KiB
ReStructuredText
.. index:: fix momentum
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fix momentum command
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====================
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fix momentum/kk 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 momentum N keyword values ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* momentum = style name of this fix command
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* N = adjust the momentum every this many timesteps
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one or more keyword/value pairs may be appended
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* keyword = *linear* or *angular* or *rescale*
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.. parsed-literal::
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*linear* values = xflag yflag zflag
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xflag,yflag,zflag = 0/1 to exclude/include each dimension
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*angular* values = none
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.. parsed-literal::
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*rescale* values = none
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix 1 all momentum 1 linear 1 1 0
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fix 1 all momentum 1 linear 1 1 1 rescale
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fix 1 all momentum 100 linear 1 1 1 angular
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Description
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"""""""""""
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Zero the linear and/or angular momentum of the group of atoms every N
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timesteps by adjusting the velocities of the atoms. One (or both) of
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the *linear* or *angular* keywords must be specified.
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If the *linear* keyword is used, the linear momentum is zeroed by
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subtracting the center-of-mass velocity of the group from each atom.
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This does not change the relative velocity of any pair of atoms. One
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or more dimensions can be excluded from this operation by setting the
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corresponding flag to 0.
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If the *angular* keyword is used, the angular momentum is zeroed by
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subtracting a rotational component from each atom.
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This command can be used to insure the entire collection of atoms (or
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a subset of them) does not drift or rotate during the simulation due
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to random perturbations (e.g. :doc:`fix langevin <fix_langevin>`
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thermostatting).
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The *rescale* keyword enables conserving the kinetic energy of the group
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of atoms by rescaling the velocities after the momentum was removed.
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Note that the :doc:`velocity <velocity>` command can be used to create
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initial velocities with zero aggregate linear and/or angular momentum.
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----------
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Styles with a *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed on the :doc:`Speed packages <Speed_packages>` doc
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page. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
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USER-OMP and OPT packages, respectively. They are only enabled if
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LAMMPS was built with those packages. See the :doc:`Build package <Build_package>` doc page for more info.
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You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the :doc:`-suffix command-line switch <Run_options>` when you invoke LAMMPS, or you can use the
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:doc:`suffix <suffix>` command in your input script.
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See the :doc:`Speed packages <Speed_packages>` doc page for more
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instructions on how to use the accelerated styles effectively.
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**Restart, fix_modify, output, run start/stop, minimize info:**
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No information about this fix is written to :doc:`binary restart files <restart>`. None of the :doc:`fix_modify <fix_modify>` options
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various :doc:`output commands <Howto_output>`.
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No parameter of this fix can be used with the *start/stop* keywords of
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the :doc:`run <run>` command. 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 recenter <fix_recenter>`, :doc:`velocity <velocity>`
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**Default:** none
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