I'm doing simulations using Guass view (Gussian) to determine the fluorescence spectrum at higher and lower PH (Deprotonation/Protonation) using DFT.
But the problem is
1: Function PBEPBE performs better at neutral PH while performing simulations at protonation/deprotonation there is a great difference observed for fluorescence spectrum as compared to neutral PH.
2: Function CAM-B3LYP performs better at higher and lower PH while performing simulations at neutral there is a great difference observed for fluorescence spectrum as compared to high/low PH.
You are facing one of the main issues of DFT: it is not systematically improvable, so one functional performing better in one case and the other in the other case is, unfortunately, not rare, and a general statement in the manner "this functional always gives correct fluorescence data" is not possible, in the best case it's something like "this functional generates reasonably interpretable fluorescence data in 70% of all tested cases while only 20% of all tested species give complete bogus".
Thank You! Jürgen Weippert for your kind response.
Is there any other way to solve this issue?
I suppose (systematically improvable) wavefunction methods are not in your computational capacity. The best advice I can give you is to dig through a large portion of literature to check for a functional/basis set combination which has produced suitable results for systems as similar as possible to yours.
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