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IBRION=5
Posted: Mon Nov 04, 2013 3:10 pm
by rogerwol
Hi.
Short question: When VASP calculates vibrational frequencies with IBRION=5, does it only include harmonic approximation, or is some anharmonic effects included in the calculations?
IBRION=5
Posted: Mon Nov 04, 2013 5:31 pm
by admin
there are no contributions from anharmonic effects, IBRION=5 assumes that one is in the harmonic regime, therefore POTIM should be chosen small (0.01 to 0.015, depending on the system of course)
IBRION=5
Posted: Mon Nov 04, 2013 6:35 pm
by rogerwol
Thanks! Great answer
IBRION=5
Posted: Tue Nov 05, 2013 9:06 am
by alex
Hi,
the calculations of anharmonic effects require 3rd derivatives of the energy wrt. coordinates. It's mostly done analytically, because of potential numerical noise.
It is not (yet) implemented in VASP (as far as I know).
Just my ?0.01
Cheers,
alex