This information is exactly what is needed to reconstruct the spectral density of molecules in solvent with full chemical complexity. In this case, the spectral density consists of a broad low-frequency feature J 0 (ω) describing the influence of solvent and a series of discrete high-frequency peaks J α (ω) (α = 1, ..., N ) due to interaction of the chromophore with intramolecular vibrational modes. The basic functional forms for the spectral density have been isolated through physical models in which the thermal environment is described as a collection of harmonic oscillators (the so-called Brownian oscillator model). This yields,