README

Program to compute the velocity-velocity autocorrelation and the
phonon density of states based on the dumped velocities from 
LAMMPS. To use this code, simply invoke:

vacf vel-dump-file-name(s)

The computed velocity-velocity autocorrelation will be written to
acf.dat, unless one specify a file name via the "-oa acf-file-name"
command line option. While the computed phonon density of states
will be written to file "dos_vacf.dat", unless one specify a different
file name via "-od dos-file-name".

The velocities should be dumped via:
dump  2  all custom n  dump-file vx vy vz
dump_modify 2 sort id

You can decompose your system into several groups and dump each
group speparately, so as to avoid super-huge files.

Note, the "dump_modify sort id" is essential to guarantee the
correctness of the acf and dos; it is also important to provide
the correct MD time step via command line option "-dt" to make
sure the units of time (in acf file) and frequency (in dos file)
to be correct. The desired unit for timestep is ps, in which
case the corresponding frequency unit will be THz. For unit
other than ps, the unit for frequency will by inverse of your
time unit.

LT Kong

konglt_at_sjtu.edu.cn

Jan 2020