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Pedro Silva
pedros@ufp.edu.pt

I have a system composed of two neutral molecules, A and B. Upon excitation with light, an electron moves from A to B. I want to characterize this charge-transfer excited state as A an B are moved toward each other, and due to the large size of the molecules I can only afford to do that with CIS.

I ran a CIS computation of the A - B system at different A-B distances, and got a sensible result in the gas phase: as the A-B distance decreases, the charge-separated excited state becomes more stable (about 30 kcal.mol-1 lowering from 100 to 8 angstrom). From inspection of the Coulomb law (ether with point charges or dipoles) I expected that in solution, this stabilization should be smaller by a factor similar to the solvent dielectric constant. However, when repeating the computation with PCM turned on I got almost the same result as in the gas phase (+/- a few kcal.mol-1). Any ideas why?

Thanks!
Pedro