The initial Γ value can be estimated from the behavior of similar molecules. The vibrational frequencies, {ω α }, are obtained from the experimental resonance Raman spectra and the Huang-Rhys factors are initially estimated under the approximation S α = Iα 2ωα where I α is the Raman integrated peak intensity. In the fitting procedure, the parameters Γ, µ, and ω eg are optimized first as they have a large and uniform effect on all three lineshapes. Then, the set {S α } is adjusted to obtain the best fit to the absorption, emission, and resonance Raman excitation profiles, while accurately reproducing the experimental Stokes shift. The inhomogeneous broadening parameter is neglected in the fitting procedure and is only introduced when a better fit cannot be achieved. The parameters obtained from the fitting procedure can be utilized to reconstruct the spectral density, employing Eq. 1 in the main text. C. Extracted spectral densities for thymine and its derivatives Tables S1-S3 summarize the intramolecular and solvent parameters needed for reconstructing the spectral density for all three molecules in Fig. . Figure details the spectral density for thymine and its nucleoside. The resonance Raman spectra for thymine was computed as described in the Materials and Methods section. The XYZ coordinates of the optimized geometries of the complexes used in the computations are shown in Table and Table . The computed frequencies and Huang-Rhys factors for the 75 normal modes in the complex are included in Table . Figure compares the computed RR spectra with the experimental results in Refs. ( ) The excellent agreement between theory and experiment enables us to assign specific vibrational modes to the peaks in the spectral density and interpret the electronic decoherence pathways. Animated Figure details representative normal modes 1-9 that most strongly couples to the electronic transition.