A quantum mechanical/molecular mechanical approach to relaxation dynamics: Calculation of the optical properties of solvated bacteriochlorophyll-a

We have applied both classical and mixed quantum mechanical/molecular mechanical (QM/MM) techniques to the calculation of electronic-vibrational coupling. In order to assess these approaches, we compare results to the steady state absorption and emission spectra of solvated bacteriochlorophyll-a (BChl-a) at room temperature. We find that the method chosen for the calculation of the S-0-S-1 I energy gap significantly affects the calculated spectra. Mixed QM/MM approaches perform substantially better than the purely classical approach, and where an ab initio method is used for calculating the S-0-S-1 energy gap, the predicted Stokes shift (related to the reorganization energy), and the spectral absorption width are within 5% of the experimental values. We fmd that the decay of the transition energy correlation function occurs largely over two time scales. Most of the decorrelation occurs in less than 5 fs. This is less than the time taken for the process of photon absorption, indicating that the optical spectrum of BChl-a in methanol is predominantly homogeneous. Moreover, we find that intramolecular dynamics of the Bchl-a affect the correlation function, with a concomitant effect on the calculated observables. This is highlighted by the presence of a Franck-Condon progression in our ab inito calculated spectra, with the effect of this progression apparently imprinted on the corresponding free energy surface.