  Representation of molecular vibrational degrees of freedom by independent
harmonic oscillators, when describing electronic spectra or electronic
excitation energy transport, raises unfavourable effects as vibrational energy
relaxation becomes inaccessible. A standard theoretical description is extended
in this paper by including both electronic-phonon and vibrational-phonon
couplings. Using this approach we have simulated a model pigment-protein system
and have shown that intermode coupling leads to the quenching of pigment
vibrational modes, and to the redistribution of fluctuation spectral density
with respect to the electronic excitations. Moreover, new energy relaxation
pathways, opened by the vibrational-phonon interaction, allow to reach the
electronic excited state equilibrium quicker in the naturally occurring water
soluble chlorophyll binding protein (WSCP) aggregate, demonstrating the
significance that the damping of molecular vibrations has for the
intrarmolecular energy relaxation process rate.
