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@ -2093,6 +2093,29 @@ print "average viscosity: $v \[Pa.s/] @ $T K, $\{ndens\} /A^3" :pre
:line
6.22 Calculating diffusion :link(howto_22),h4
The diffusion coefficient D of a material can be measured in at least
2 ways using various options in LAMMPS. See the examples/DIFFUSE
directory for scripts that implement the 2 methods discussed here for
a simple Lennard-Jones fluid model.
The first method is to measure the mean-squared displacement (MSD) of
the system, via the "compute msd"_compute_msd.html command. The slope
of the MSD versus time is proportional to the diffusion coefficient.
The instantaneous MSD values can be accumulated in a vector via the
"fix vector"_fix_vector.html command, and a line fit to the vector to
compute its slope via the "variable slope"_variable.html function, and
thus extract D.
The second method is to measure the velocity auto-correlation function
(VACF) of the system, via the "compute vacf"_compute_vacf.html
command. The time-integral of the VACF is proportional to the
diffusion coefficient. The instantaneous VACF values can be
accumulated in a vector via the "fix vector"_fix_vector.html command,
and time integrated via the "variable trap"_variable.html function,
and thus extract D.
:line
:line
@ -2129,28 +2152,3 @@ Phys, 79, 926 (1983).
:link(Shinoda)
[(Shinoda)] Shinoda, Shiga, and Mikami, Phys Rev B, 69, 134103 (2004).
:line
6.22 Calculating diffusion :link(howto_22),h4
The diffusion coefficient D of a material can be measured in at least
2 ways using various options in LAMMPS. See the examples/DIFFUSE
directory for scripts that implement the 2 methods discussed here for
a simple Lennard-Jones fluid model.
The first method is to measure the mean-squared displacement (MSD) of
the system, via the "compute msd"_compute_msd.html command. The slope
of the MSD versus time is proportional to the diffusion coefficient.
The instantaneous MSD values can be accumulated in a vector via the
"fix vector"_fix_vector.html command, and a line fit to the vector to
compute its slope via the "variable slope"_variable.html function, and
thus extract D.
The second method is to measure the velocity auto-correlation function
(VACF) of the system, via the "compute vacf"_compute_vacf.html
command. The time-integral of the VACF is proportional to the
diffusion coefficient. The instantaneous VACF values can be
accumulated in a vector via the "fix vector"_fix_vector.html command,
and time integrated via the "variable trap"_variable.html function,
and thus extract D.