In the condensed phase systems that form the focus of this work, an additional approximation has to be introduced, as the exact quantum autocorrelation function is generally inaccessible. A common strategy is based on using the purely classical autocorrelation function, which is generally much easier to calculate, to approximately reconstruct its quantum counterpart using quantum correction factors. While the choice of correction factor is not unique, in this work we make use of the harmonic quantum correction factor throughout. The spectral density can then be written in terms of the classical autocorrelation function as: