Excitation energies of retinal chromophores: critical role of the structural model.

We employ a variety of highly-correlated approaches including quantum Monte Carlo (QMC) and the n-electron valence state perturbation theory (NEVPT2) to compute the vertical excitation energies of retinal protonated Schiff base (RPSB) models in the gas phase. We find that the NEVPT2 excitation energies are in good agreement with the QMC values and confirm our previous findings that the complete-active-space perturbation (CASPT2) approach yields accurate excitations for RPSB models only when the more recent zero-order IPEA Hamiltonian is employed. The excitations computed with the original zero-order formulation of CASPT2 are instead systematically red-shifted by more than 0.3 eV. We then focus on the full 11-cis retinal chromophore and show that the M06-2X and MP2 approaches provide reliable ground-state equilibrium structures for this system while the complete-active-space self-consistent field (CASSCF) geometry is characterized by significantly higher ground-state energies at the NEVPT2 and CASPT2 level. Our calibration of the structural model together with the general agreement of all highly-correlated excited-state methods allows us to reliably assign a value of about 2.3 eV to the vertical excitation of 11-cis RPSB in the gas-phase.