We investigate the optical properties of the tZt-penta-3,5-dieniminium cation, a simpliﬁed model for the protonated Schiff base of 11-cis retinal in rhodopsin, along the isomerization pathway by ab-initio calculations based on Many-Body Perturbation Theory using the GW method and the Bethe–Salpeter equation. Our calculations are carried out on a few signiﬁcant CASSCF geometrical conﬁgurations of the isomerization minimal energy path taken from the literature. Our excitation energies are qualitatively in agreement with previous Quantum Monte Carlo and post-Hartree–Fock calculations. We also employ TDDFT based methods, and investigate the outcome of using different approximations and several exchange– correlation functionals.
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