104 lines
3.3 KiB
ReStructuredText
104 lines
3.3 KiB
ReStructuredText
.. index:: fix spring/self
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.. index:: fix spring/self/kk
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fix spring/self 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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fix ID group-ID spring/self K dir
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* spring/self = style name of this fix command
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* K = spring constant (force/distance units)
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* dir = xyz, xy, xz, yz, x, y, or z (optional, default: xyz)
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix tether boundary-atoms spring/self 10.0
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fix zrest move spring/self 10.0 z
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Description
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"""""""""""
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Apply a spring force independently to each atom in the group to tether
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it to its initial position. The initial position for each atom is its
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location at the time the fix command was issued. At each timestep,
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the magnitude of the force on each atom is -Kr, where r is the
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displacement of the atom from its current position to its initial
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position. The distance r correctly takes into account any crossings
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of periodic boundary by the atom since it was in its initial
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position.
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With the (optional) dir flag, one can select in which direction the
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spring force is applied. By default, the restraint is applied in all
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directions, but it can be limited to the xy-, xz-, yz-plane and the
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x-, y-, or z-direction, thus restraining the atoms to a line or a
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plane, respectively.
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Restart, fix_modify, output, run start/stop, minimize info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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This fix writes the original coordinates of tethered atoms to
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:doc:`binary restart files <restart>`, so that the spring effect will
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be the same in a restarted simulation. See the :doc:`read_restart
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<read_restart>` command for info on how to re-specify a fix in an
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input script that reads a restart file, so that the operation of the
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fix continues in an uninterrupted fashion.
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The :doc:`fix_modify <fix_modify>` *energy* option is supported by
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this fix to add the energy stored in the per-atom springs to the
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global potential energy of the system as part of :doc:`thermodynamic
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output <thermo_style>`. The default setting for this fix is
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:doc:`fix_modify energy no <fix_modify>`.
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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 forces. Default is the outermost level.
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This fix computes a global scalar which can be accessed by various
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:doc:`output commands <Howto_output>`. The scalar is an energy which is
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the sum of the spring energy for each atom, where the per-atom energy
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is 0.5 \* K \* r\^2. The scalar value calculated by this fix is
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"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 per-atom spring energy to be included in the
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total potential energy of the system (the quantity being minimized),
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you MUST enable the :doc:`fix_modify <fix_modify>` *energy* option for
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this fix.
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----------
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.. include:: accel_styles.rst
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----------
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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 drag <fix_drag>`, :doc:`fix spring <fix_spring>`,
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:doc:`fix smd <fix_smd>`, :doc:`fix spring/rg <fix_spring_rg>`
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Default
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"""""""
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none
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