| Abstract
| - In this paper we use ab initio multireference Møller−Plesset second-order perturbation theorycomputations to map the first five singlet states (S0, S1, S2, S3, and S4) along the initial part of thephotoisomerization coordinate for the isolated rhodopsin chromophore model 4-cis-γ-methylnona-2,4,6,8-tetraeniminium cation. We show that this information not only provides an explanation for the spectralfeatures associated to the chromophore in solution but also, subject to a tentative hypothesis on the effectof the protein cavity, may be employed to explain/assign the ultrafast near-IR excited-state absorption,stimulated emission, and transient excited-state absorption bands observed in rhodopsin proteins (e.g.rhodopsin and bacteriorhodopsin). We also show that the results of vibrational frequency computationsreveal a general structure for the first (S1) excited-state energy surface of PSBs that is consistent with theexistence of the coherent oscillatory motions observed both in solution and in bacteriorhodopsin.
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