Scans on the excited state PES, search of the conical intersection points and calculations of the vertical excitation energies and oscillator strengths were carried out for a large molecular cluster, which was almost identical to the QM subsystem in QM/MM simulations in the fullprotein model. Fig. shows this cluster in the configuration of the ON-state. All calculations for cluster models were performed with the Firefly package . The state-averaged complete active space self-consistent field (CASSCF) method with the distribution of 8 electrons over 6 complete active orbitals represented by the 6-31G basis set (SA2-CASSCF(8/6)/6-31G) was used to locate structures on the excited state PES and the minimum energy conical intersection points. Calculations of the vertical excitation energies and oscillator strengths of electronic transitions were carried out with the algorithms of the extended multiconfigurational quasidegenerate perturbation theory in the second order (XMCQDPT2) based on the SA-CASSCF(16/12)/cc-pVDZ wavefunctions as in Ref. .