Diabatic states of a photoexcited retinal chromophore from ab initio many-body perturbation theory

We investigate the diabatic electronic states of a photoexcited molecule within a diabatization method originally proposed by Baer [Chem. Phys. Lett. 35, 112 (1975)]. A diabatization denotes a unitary transformation which allows for incorporating nonadiabatic effects into the quantum Hamiltonian, expressed in the adiabatic representation. A typical example is the treatment of avoided crossings in the potential-energy surface, for instance, in the case of the retinal chromophore. In this paper, we present analytical and numerical calculations for the diabatic states in the context of Green's-function-based ab initio many-body perturbation theory (density-functional theory plus GW method plus Bethe-Salpeter equation). We present the calculation of the adiabatic and diabatic lowest excited electronic states of the retinal chromophore molecule.