115 lines
3.7 KiB
ReStructuredText
115 lines
3.7 KiB
ReStructuredText
.. index:: fix addtorque
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fix addtorque 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 addtorque Tx Ty Tz
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* addtorque = style name of this fix command
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* Tx,Ty,Tz = torque component values (torque units)
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* any of Tx,Ty,Tz can be a variable (see below)
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix kick bead addtorque 2.0 3.0 5.0
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fix kick bead addtorque 0.0 0.0 v_oscillate
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Description
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"""""""""""
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Add a set of forces to each atom in
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the group such that:
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* the components of the total torque applied on the group (around its
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center of mass) are Tx,Ty,Tz
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* the group would move as a rigid body in the absence of other
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forces.
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This command can be used to drive a group of atoms into rotation.
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Any of the 3 quantities defining the torque components can be specified
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as an equal-style :doc:`variable <variable>`, namely *Tx*,
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*Ty*, *Tz*\ . If the value is a variable, it should be specified as
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v_name, where name is the variable name. In this case, the variable
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will be evaluated each timestep, and its value used to determine the
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torque component.
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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 torque.
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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>` *energy* option is supported by
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this fix to add the potential "energy" inferred by the added torques
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to the global potential energy of the system as part of
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:doc:`thermodynamic output <thermo_style>`. The default setting for
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this fix is :doc:`fix_modify energy no <fix_modify>`. Note that this
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is a fictitious quantity but is needed so that the :doc:`minimize
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<minimize>` command can include the forces added by this fix in a
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consistent manner. I.e. there is a decrease in potential energy when
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atoms move in the direction of the added forces.
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The :doc:`fix_modify <fix_modify>` *respa* option is supported by
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this fix. This allows to set at which level of the :doc:`r-RESPA <run_style>`
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integrator the fix is adding its torque. Default is the outermost level.
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This fix computes a global scalar and a global 3-vector, which can be
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accessed by various :doc:`output commands <Howto_output>`. The scalar
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is the potential energy discussed above. The vector is the total
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torque on the group of atoms before the forces on individual atoms are
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changed by the fix. The scalar and vector values calculated by this
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fix are "extensive".
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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.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the :doc:`minimize <minimize>` command.
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.. note::
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If you want the fictitious potential energy associated with the
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added forces to be included in the total potential energy of the
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system (the quantity being minimized), you MUST enable the
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:doc:`fix_modify <fix_modify>` *energy* option for this fix.
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.. note::
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You should not specify force components with a variable that has
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time-dependence for use with a minimizer, since the minimizer
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increments the timestep as the iteration count during the
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minimization.
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Restrictions
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""""""""""""
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This fix is part of the MISC package. It is only enabled if
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LAMMPS was built with that package. See the :doc:`Build package
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<Build_package>` page for more info.
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Related commands
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""""""""""""""""
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:doc:`fix addforce <fix_addforce>`
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Default
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"""""""
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none
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