85 lines
2.5 KiB
ReStructuredText
85 lines
2.5 KiB
ReStructuredText
.. index:: compute vacf
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compute vacf 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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compute ID group-ID vacf
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* vacf = style name of this compute command
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Examples
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""""""""
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.. code-block:: LAMMPS
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compute 1 all vacf
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compute 1 upper vacf
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Description
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"""""""""""
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Define a computation that calculates the velocity auto-correlation
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function (VACF), averaged over a group of atoms. Each atom's
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contribution to the VACF is its current velocity vector dotted into
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its initial velocity vector at the time the compute was specified.
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A vector of four quantities is calculated by this compute. The first three
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elements of the vector are :math:`v_x v_{x,0}` (and similar for the
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:math:`y` and :math:`z` components), summed and averaged over atoms in the
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group, where :math:`v_x` is the current :math:`x`-component of the velocity of
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the atom and :math:`v_{x,0}` is the initial :math:`x`-component of the velocity
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of the atom. The fourth element of the vector is the total VACF
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(i.e., :math:`(v_x v_{x,0} + v_y v_{y,0} + v_z v_{z,0})`),
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summed and averaged over atoms in the group.
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The integral of the VACF versus time is proportional to the diffusion
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coefficient of the diffusing atoms. This can be computed in the
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following manner, using the :doc:`variable trap() <variable>` function:
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.. code-block:: LAMMPS
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compute 2 all vacf
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fix 5 all vector 1 c_2[4]
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variable diff equal dt*trap(f_5)
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thermo_style custom step v_diff
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.. note::
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If you want the quantities calculated by this compute to be
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continuous when running from a :doc:`restart file <read_restart>`, then
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you should use the same ID for this compute, as in the original run.
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This is so that the fix this compute creates to store per-atom
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quantities will also have the same ID, and thus be initialized
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correctly with time=0 atom velocities from the restart file.
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Output info
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"""""""""""
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This compute calculates a global vector of length 4, which can be
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accessed by indices 1--4 by any command that uses global vector values
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from a compute as input. See the :doc:`Howto output <Howto_output>` doc
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page for an overview of LAMMPS output options.
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The vector values are "intensive". The vector values will be in
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velocity\ :math:`^2` :doc:`units <units>`.
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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:`compute msd <compute_msd>`, :doc:`compute vacf/chunk <compute_vacf_chunk>`
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Default
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"""""""
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none
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