Modelling of photochemical reaction paths, including electronic state crossings, often requires to rely on highly flexible wavefunction-based theories featuring a balanced description of static and dynamic electron correlation. Such approaches are often computationally expensive, precluding a complete analysis of the photochemical properties of the studied system. Moreover, in photoactive biomolecular systems, the spatial extension of the subsystem engaged in the photochemical process prompts for the use of cost-effective approaches. For instance, upon Ca2+ discharge, the obelin oxidized substrate transforms into the coelenteramide luminophore in a singlet excited state, whose radiative decay to the ground state is possible thanks to a bimodal light emission. More precisely, excited coelenteramide exists in two protonation states, neutral and anionic, the latter being associated with an excited state proton transfer to the close His22, leading to an ion-pair (Figure 1). Such an ion-pair is not supported by calculations rooted in time-dependent Kohn-Sham Density Functional Theory (DFT). Moreover, an alternative non-radiative decay mediated by a state crossing is found. Similarly, the photoisomerization path of the retinal chromophore in rhodopsin proteins involves a conical intersection, and generally large portions of quasi-degenerate potential energy surfaces. We assess the performances of several flavors of DFT-based methods (including time-dependent and restricted ensemble Kohn-Sham) in the region of near-degeneracy of a retinal model.

Approximate density functional theory for complex photoreactions in biological systems

247th National Meeting (2014)

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