24 lines
1.2 KiB
ReStructuredText
24 lines
1.2 KiB
ReStructuredText
Calculate diffusion coefficients
|
|
================================
|
|
|
|
The diffusion coefficient :math:`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 :doc:`compute msd <compute_msd>` 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
|
|
:doc:`fix vector <fix_vector>` command, and a line fit to the vector to
|
|
compute its slope via the :doc:`variable slope <variable>` function, and
|
|
thus extract :math:`D`.
|
|
|
|
The second method is to measure the velocity auto-correlation function
|
|
(VACF) of the system, via the :doc:`compute vacf <compute_vacf>`
|
|
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 :doc:`fix vector <fix_vector>` command,
|
|
and time integrated via the :doc:`variable trap <variable>` function,
|
|
and thus extract :math:`D`.
|