Absorption spectrum of the Green Fluorescent Protein chromphore: A difficult case for ab into methods?

We perform a thorough comparative investigation of the excitation energies of the anionic and neutral forms of the green fluorescent protein (GFP) chromophore in the gas phase using a variety of first-principle theoretical approaches commonly used to access excited state properties of photoactive molecules. These include time-dependent density functional theory (TDDFT), complete-active-space second-order perturbation theory (CASPT2), equation-of-motion coupled cluster (EOM-CC), and quantum Monte Carlo (QMC) methods. We find that all approaches give roughly the same vertical excitation for the anionic form, while TDDFT predicts an excitation for the neutral chromophore significantly lower than the highly correlated methods. Our findings support the picture emerging from the extrapolation of the Kamlet-Taft fit of absorption experimental data in solution and indicate that the protein gives rise to a considerable bathochromic shift with respect to vacuum. These results also open some questions on the interpretation of photodestruction spectroscopy experiments in the gas phase as well as on the accuracy of previous theoretical calculations in the more complex protein environment