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Question about analytical/numerical DFT hessian

Marco Lombardo

Dear Firefly users,
first of all I'm not a computational chemist and I'm just beginning to learn Firefly. Back to the question, using a Core i7 CPU on Windows 7, I was able to optimize two organic reactants systems composed of 85 atoms (no metals) using B3LYP/6-31G(d) and to find the two correspondig transition states at the same level of theory. Each work takes 4 to 8 hours to complete and I think that these are quite impressive results (using this hardware) in terms of computational times.
Now, if I want to check I have minima and real TSs, I have also to run a HESSIAN calculation for each structure. Using DFT this kind of calculation (only METHOD=NUMERIC is available) takes more than 24 hours for each structure! (85*3 = 255 single point energies have to be computed and each one needs around 6-8 minutes... this means 25.5-34 hours).
I read somewhere in the forum that analytical DFT gradient method is planned, but it will (probably) not be present in the (near) future 8 release, since it is not a real priority giving only a rather small gain in computational times.
So, since frequencies have to be computed in order to confirm stationary points or saddle points on the PES, which is the best strategy to save time? I think there is no much advantage to gain 4-8 hours (with respect to other computational softwares) in the optimization steps only to be slow down in the next ones.
That said, Firefly is really a nice piece of software!


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