Accurate modeling of the S0-S1 photo-absorption in biological chromophores

We address the problem of quantitative evaluation of the absorption S₀-S₁ peaks &lgr; _max of biological chromophores in vacuo by using the state-of-art computational methods of quantum theory. In particularly, we rely on the second order multiconfigurational quasidegenerate perturbation theory (MCQDPT2) following the complete active space selfconsistent field (CASSCF) calculations. The use of augmented effective Hamiltonian operators in the MCQDPT2 framework allows us to correct deficiencies of the standard multistate approaches and to obtain stable saturated solutions for the target low-lying excited states. A high accuracy of the proposed methodology is illustrated for several photoactive protein chromophores in the gas phase including all-trans retinal, green fluorescent protein type chromophores and photoactive yellow protein chromophores. It is shown that our approach provides correct ordering of states and predicts maxima of absorption bands for the S₀-S₁ transitions within only a few nanometers from experimental data.