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33 lines
1.2 KiB
ReStructuredText
33 lines
1.2 KiB
ReStructuredText
Calculate diffusion coefficients
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The diffusion coefficient D of a material can be measured in at least
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2 ways using various options in LAMMPS. See the examples/DIFFUSE
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directory for scripts that implement the 2 methods discussed here for
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a simple Lennard-Jones fluid model.
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The first method is to measure the mean-squared displacement (MSD) of
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the system, via the :doc:`compute msd <compute_msd>` command. The slope
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of the MSD versus time is proportional to the diffusion coefficient.
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The instantaneous MSD values can be accumulated in a vector via the
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:doc:`fix vector <fix_vector>` command, and a line fit to the vector to
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compute its slope via the :doc:`variable slope <variable>` function, and
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thus extract D.
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The second method is to measure the velocity auto-correlation function
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(VACF) of the system, via the :doc:`compute vacf <compute_vacf>`
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command. The time-integral of the VACF is proportional to the
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diffusion coefficient. The instantaneous VACF values can be
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accumulated in a vector via the :doc:`fix vector <fix_vector>` command,
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and time integrated via the :doc:`variable trap <variable>` function,
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and thus extract D.
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----------
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Commands_all.html
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