IBRION=5

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rogerwol
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IBRION=5

#1 Post by rogerwol » Mon Nov 04, 2013 3:10 pm

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?
Last edited by rogerwol on Mon Nov 04, 2013 3:10 pm, edited 1 time in total.
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admin
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IBRION=5

#2 Post by admin » Mon Nov 04, 2013 5:31 pm

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)
Last edited by admin on Mon Nov 04, 2013 5:31 pm, edited 1 time in total.
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rogerwol
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IBRION=5

#3 Post by rogerwol » Mon Nov 04, 2013 6:35 pm

Thanks! Great answer :)
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alex
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IBRION=5

#4 Post by alex » Tue Nov 05, 2013 9:06 am

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
Last edited by alex on Tue Nov 05, 2013 9:06 am, edited 1 time in total.
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