Is there molecule size limit on vibrational averaging?

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Is there molecule size limit on vibrational averaging?

Post by afca » 11 Mar 2016, 04:09


Does anyone have any experience with the calculation of vibrational averaged electric properties of large molecules? For example, is possible to evaluate vibrational averaged properties to a fullerene molecule using B3LYP/6-31G** or similar theory level? Is it just a matter of computational capacity or the program has some limitation on vibrational averaging to large systems?

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Re: Is there molecule size limit on vibrational averaging?

Post by kennethruud » 11 Mar 2016, 07:25


There is in principle no limitations apart from computer time on the size of the system you can calculate, and should you hit any particular preset limits in the code, these limits can be reset (but for C60 I do not think you will hit any such limits).

I could add that we have a code for calculating the necessary ingredients for calculating vibrationally averaged electric properties analytically, and we are working on improving the efficiency of the code. This code we hope to release by summer or early in the autumn.

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Re: Is there molecule size limit on vibrational averaging?

Post by taylor » 11 Mar 2016, 12:52

Although the program can likely handle the calculation you propose, I fear that using B3LYP/6-31G** will mean you will waste a great deal of computer time getting largely worthless results. First, the basis set is rubbish: it is not even split-valence quality in the valence sp space. Second, B3LYP was parametrized (admittedly using this basis) for energies. Since DFT is a semiempirical method there is no reason, only hope, that it will give good properties. Certainly if you are interested in electric properties I don't think anyone would have any faith in it. Bear in mind if you go ahead you will be looking to calculate effective geometries and force constants for a 60-atom molecule with 15 (the original set was optimized using Cartesian d functions) basis functions per atom in no symmetry. 900 basis functions, no symmetry, and a functional that cannot be relied on for any property other than the energy, except for fortuitous agreement. This does not seem very profitable as a use of computer resources!

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