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Re: Question about analytical/numerical DFT hessian

Solntsev Pasha

Dear Marco.
As i understand correctly, you don't have enough time to finish you HESSIAN job. If so, check IRCDATA file from aborted job. You need transfer part $VIB into input file and restart your job. Before i thought it is disadvantage of FF but actually it is very nice ability, because you can step-by-step get all modes even if you have big system and very small amount of computational time.

All the best.


On Mon Dec 19 '11 2:09pm, Marco Lombardo wrote
>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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