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Hi,

I would like to estimate the reaction rate constant for water dissociation over nickel surface. I have found the TST and several soft modes after frequency calculations and I am afraid if harmonic oscillator approximation fails in this case. I was wondering if I can use k=(kbT/h)exp(-∆G#/RT) approximation.
If yes, how can I find the entropy of the transition state (one needs the equation: ∆G#=∆H#-T∆S# where ∆H#=Htst-Hreact etc.)? Is the entropy written in OUTCAR? I prefer not to use vibrational frequencies because of soft modes.
If not,does any body know what method I can use?
It would be appreciated if some one could help me.

Regards
Abas
[ Edited ]

There is no entropy written in OUTCAR. Free energy can be obtained using constrained MD. MD runs for a series of constrained geometries provide free energy gradients (derivatives of the free energy). Numerical integration of gradients along the reaction path gives you Delta G. Cf e.g. J. Catal. 298(2013)122.
By the way H2O on Ni is rather difficult case for constrained MD.

Thanks for the tips!

Regards

abas