I think that if there are two unstable directions at the saddle point this means there is another saddle point with a lower Activation energy nearby. Just slide the neb downhill stefano
Sent from my Samsung Galaxy smartphone. -------- Original message --------From: Pascal Boulet <[email protected]> Date: 13/11/2017 19:45 (GMT+01:00) To: PWSCF Forum <[email protected]> Subject: *spam*Re: [Pw_forum] Physical meaning of two imaginary phonon frequencies at NEB saddle point and Transition State Theory Hello, My guess: you have to remove one of the imaginary frequencies (most probably the smallest one) by following the corresponding eigenvector (=moving the atoms accordingly). Then, you will have to make sure that the transition state connects the reactants and products you are interested in. HTH,Pascal Pascal Boulet—Professor in computational chemistry - DEPARTEMENT OF CHEMISTRYAix-Marseille University - ST JEROME - Avenue Escadrille Normandie Niemen - F-13013 Marseille - FRANCETél: +33(0)4 13 55 18 10 - Fax : +33(0)4 13 55 18 50Site : http://madirel.univ-amu.fr/pages_web_BOULET_PASCAL/infos - Email : [email protected] Le 13 nov. 2017 à 12:44, JAY Antoine <[email protected]> a écrit : 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 _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
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