92 lines
2.4 KiB
ReStructuredText
92 lines
2.4 KiB
ReStructuredText
.. index:: fix rhok
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fix rhok 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 rhok nx ny nz K a
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* nx, ny, nz = k-vector of collective density field
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* K = spring constant of bias potential
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* a = anchor point of bias potential
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix bias all rhok 16 0 0 4.0 16.0
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fix 1 all npt temp 0.8 0.8 4.0 z 2.2 2.2 8.0
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# output of 4 values from fix rhok: U_bias rho_k_RE rho_k_IM \|rho_k\|
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thermo_style custom step temp pzz lz f_bias f_bias[1] f_bias[2] f_bias[3]
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Description
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"""""""""""
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The fix applies a force to atoms given by the potential
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.. math::
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U = & \frac{1}{2} K (|\rho_{\vec{k}}| - a)^2 \\
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\rho_{\vec{k}} = & \sum_j^N \exp(-i\vec{k} \cdot \vec{r}_j )/\sqrt{N} \\
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\vec{k} = & (2\pi n_x /L_x , 2\pi n_y /L_y , 2\pi n_z/L_z )
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as described in :ref:`(Pedersen) <Pedersen>`.
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This field, which biases configurations with long-range order, can be
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used to study crystal-liquid interfaces and determine melting
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temperatures :ref:`(Pedersen) <Pedersen>`.
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An example of using the interface pinning method is located in the
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*examples/PACKAGES/rhok* directory.
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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
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<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 calculated by the fix 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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This fix computes a global scalar which can be accessed by various
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:doc:`output commands <Howto_output>`. The scalar is the potential
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energy discussed in the preceding paragraph. The scalar stored by
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this fix is "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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This fix is not invoked during :doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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This fix is part of the EXTRA-FIX 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:`thermo_style <thermo_style>`
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Default
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"""""""
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none
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----------
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.. _Pedersen:
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**(Pedersen)** Pedersen, J. Chem. Phys., 139, 104102 (2013).
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