Modeling of the structure and electronic spectra of green fluorescent protein chromophore

The structure and electronic absorption spectra of a model green fluorescent protein chromophore were studied in the neutral, cationic, and anionic forms in the isolated state. The energies of S0-S1 vertical transitions were calculated using a new method based on a modified multiconfiguration quasi-degenerate perturbation theory (aug-MCQDPT2). This method was used to obtain quantitative estimates of S0-S1 vertical transition energies for chromophores in the isolated state, 2.54, 3.12, and 3.11 eV (the experimental values were 2.59, 3.05, and 2.99 eV) for the anionic, cationic, and neutral forms, respectively.