[Pw_forum] Physical meaning of two imaginary phonon frequencies at NEB saddle point

[JAY Antoine]

Dear all,
I have been using NEB to obtain the Delta E and phonon DOS at begining/end/saddle point to properly evaluate the vibrationnal entropy contribution in the transition state theory.
Commonly, one frequency in "negative" at the saddle point, the one that is "responssible" of the transition and the following of its eigen vector from the initial state give the minimum energy path.
However, when two phonon frequencies are "negatives" at the saddle point, that means that at least 3  energy minima are link together to the same saddle point.
The transition state theory cannot be applied there as the preexponential term is the ratio of the 3N-3 phonon frequencies of initial state over the 3N-4 frequencies of the saddle point, giving the dimension of a time (state life time)
Using 3N-5 frequencies without the two negatives ones would not have a physical meaning for the initial partition function.

Any idea on how to treat such a problem to evaluate the transition rate would be appreciated.

Best regards.

Antoine Jay
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