34 lines
1.3 KiB
Plaintext
34 lines
1.3 KiB
Plaintext
"Higher level section"_Howto.html - "LAMMPS WWW Site"_lws - "LAMMPS
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Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Commands_all.html)
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:line
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Calculate diffusion coefficients :h3
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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 "compute msd"_compute_msd.html 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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"fix vector"_fix_vector.html command, and a line fit to the vector to
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compute its slope via the "variable slope"_variable.html 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 "compute vacf"_compute_vacf.html
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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 "fix vector"_fix_vector.html command,
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and time integrated via the "variable trap"_variable.html function,
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and thus extract D.
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:line
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