Аннотация:We report the results of quantum mechanical/molecular mechanical (QM/MM) calculations of the structure and spectra of bathorhodopsin which is a stable intermediate in the photocycle of the visual pigment rhodopsin. Starting from the coordinates of heavy atoms in the recently resolved crystallographic structure of bathorhodopsin (PDBID: 2G87) the equilibrium geometry parameters and vibrational frequencies were calculated by using the density functional theory PBE0/cc-pVDZ approximation in the QM part and the AMBER force field parameters in the MM part. Vertical transition energy of the S(0)-S(1) electronic excitation was estimated in the multiconfigurational quasidegenerate perturbation theory following the state averaged CASSCF(12,12)/cc-pVDZ calculations with a careful selection of sequentially augmented effective Hamiltonian matrices. The finally reported value of the S(0)-S(1) optical band is 519 nm that almost perfectly matches the experimental measurements. Calculated vibrational spectra of both model protein systems compared here, bathorhodopsin and rhodopsin, cover three main regions referring to the hydrogen-out-of-plan (HOOP) motion, the C=C ethylenic stretches, and the C-C single-bond stretches.