Hello all,

Can we calculate frequency for a minimum energy crossing point structure (mecp) in Qchem?

I did this and also got the frequency, I want to see if this frequency can be trusted?

You can compute and diagonalize the Hessian at any point that you like, and that’s what JOBTYPE=FREQ does, but the eigenvalues of the Hessian are only interpretable as vibrational frequencies if the point is a local minimum (zero gradient), which an MECP is not. Therefore the Hessian tells you about curvature of the surface but not in a way that’s easy to read off and interpret as you are trying to do.

Hello John,

Thanks for your reply, I want to see, is it possible to use the free energy from a MECP frequency to compare with other structures free energy in an energy profile of a reaction? And that which multiplicity is included in the frequency calculation in a MECP calculation. For example for S1 to T1. Since the frequency results with singlet and triplet multiplicity will give different results and I do not know which process is going to consider both multiplicities.

I do not think it is that straightforward to get free energy (i.e., finite-temperature) corrections at a MECP…