80 lines
2.1 KiB
ReStructuredText
80 lines
2.1 KiB
ReStructuredText
.. index:: third_order
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.. index:: third_order/kk
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third_order command
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===================
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Accelerator Variant: third_order/kk
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Syntax
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""""""
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.. code-block:: LAMMPS
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third_order group-ID style delta args keyword value ...
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* group-ID = ID of group of atoms to displace
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* style = *regular* or *eskm*
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* delta = finite different displacement length (distance units)
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* one or more keyword/arg pairs may be appended
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.. parsed-literal::
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keyword = *file* or *binary*
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*file* name = name of output file for the third order tensor
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*binary* arg = *yes* or *no* or *gzip*
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Examples
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""""""""
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.. code-block:: LAMMPS
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third_order 1 regular 0.000001
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third_order 1 eskm 0.000001
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third_order 3 regular 0.00004 file third_order.dat
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third_order 5 eskm 0.00000001 file third_order.dat binary yes
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Description
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"""""""""""
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Calculate the third order force constant tensor by finite difference of the selected group,
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.. math::
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\Phi^{\alpha\beta\gamma}_{ijk} = \frac{\partial^3 U}{\partial x_{i,\alpha} \partial x_{j,\beta} \partial x_{k, \gamma}}
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where Phi is the third order force constant tensor.
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The output of the command is the tensor, three elements at a time. The
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three elements correspond to the three gamma elements for a specific i/alpha/j/beta/k.
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The initial five numbers are i, alpha, j, beta, and k respectively.
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If the style eskm is selected, the tensor will be using energy units of 10 J/mol.
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These units conform to eskm style from the dynamical_matrix command, which
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will simplify operations using dynamical matrices with third order tensors.
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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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The command collects a 9 times the number of atoms in the group on every single MPI rank,
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so the memory requirements can be very significant for large systems.
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This command is part of the PHONON package. It is only enabled if
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LAMMPS was built with that package. See the :doc:`Build package <Build_package>` page for more info.
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Related commands
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""""""""""""""""
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:doc:`fix phonon <fix_phonon>` :doc:`dynamical_matrix <dynamical_matrix>`
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Default
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"""""""
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The default settings are file = "third_order.dat", binary = no
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