Quantum chemical vibrational analysis of the chromophore of bacteriorhodopsin

Vibrational spectra of the retinal chromophore in bacteriorhodopsin (BR) are interpreted by a quantum chemical vibrational analysis. The chromophore is represented by a semiempirical MNDO Hamiltonian [modified neglect of diatomic overlap] that describes all valence electrons using Hartree-Fock wave functions. The main sources of errors in a description based on the MNDO force field are identified. We show that the errors are mainly due to the neglect of the effects of electron correlation. A proper understanding of vibrational spectra of the BR chromophore can be obtained only if effects of electron correlation are taken into account.