Quantum chemical comparison of vertical, adiabatic, and 0-0 excitation energies: the PYP and GFP chromophores.

A number of benchmark studies investigating the performance of quantum chemical methods for calculating vertical excitation energies are today available in the literature. However, less established is the variation between methods in their estimates of the differences between vertical, adiabatic, and 0-0 excitation energies. To this end, such excitation energies are here calculated for the bright S(1) states of the anionic chromophores of the photoactive yellow protein (PYP) and the green fluorescent protein (GFP) in the gas phase using configuration interaction singles, complete active space self-consistent field, coupled-cluster singles and doubles, and time-dependent density functional theory methods. Although the estimates of the excitation energies vary by more than 1 eV between the methods, the differences between the different types of excitation energies are found to be relatively method-insensitive, varying by ~0.1 eV only for these particular chromophores. Specifically, the adiabatic energies are uniformly 0.10-0.17 (PYP) and 0.06-0.17 eV (GFP) lower than the vertical energies, and the 0-0 energies are similarly 0.09-0.14 (PYP) and 0.07-0.17 eV (GFP) lower than the adiabatic energies.