Tim's paper on combining the Ensemble and Franck-Condon approaches for modeling absorption spectra has been accepted in the Journal of Chemical Physics and has been highlighted as an Editor's Pick!
In this manuscript, we present an approach for modeling the band shapes of electronic absorption spectra of molecules in solution that combines both ensemble sampling of solute-solvent configurations and vibronic effects. For dyes in solution, capturing both inhomogeneous broadening of the solvent environment (which may be non-Gaussian for strong solute-solvent interactions) and the contribution to broadening from vibronic transitions is necessary for obtaining the correct spectral shape. Our approach treats all temperature effects classically through ensemble sampling of the ground state potential energy surface (including anharmonic regions of the PES), whereas the vibronic fine structure resulting from nuclear wave functions is treated at 0K through the Franck-Condon principle. These two approaches are then combined by dressing the vertical excitation energies with the Franck-Condon vibronic shape function.
We take into account the contribution of solute-solvent interactions to the absorption spectrum in three ways: 1) the configurations (sampled from MD) allow the full space of solute-solvent geometries to be explored, capturing non-Gaussian inhomogeneous broadening effects, 2) a large QM region including approximately a full solvent shell is included in the computation of the vertical excitation energies, allowing for polarization and charge-transfer between solute and solvent, 3) a small amount of frozen solvent + implicit solvent is included in the Franck-Condon computations in order to generate an average vibronic shape function. We give an overview of the advantages and disadvantages of both ensemble and Franck-Condon spectral shape approaches, outline the theory of the combined approach, and then apply the method to simulate the absorption spectra of the GFP and Nile Red chromophores in different solvents. Significant improvement is achieved in agreement with experimental spectra shapes with the combined approach compared to the ensemble approach. We believe this work provides a promising approach for straight-forwardly computing accurate spectral shapes for molecules in solution.