  We present a scheme to express a bath correlation function (BCF)
corresponding to a given spectral density (SD) as a sum of damped harmonic
oscillations. Such a representation is needed, for example, in many open
quantum system approaches. To this end we introduce a class of fit functions
that enables us to model ohmic as well as superohmic behavior. We show that
these functions allow for an analytic calculation of the BCF using pole
expansions of the temperature dependent hyperbolic cotangent. We demonstrate
how to use these functions to fit spectral densities exemplarily for cases
encountered in the description of photosynthetic light harvesting complexes.
Finally, we compare absorption spectra obtained for different fits with exact
spectra and show that it is crucial to take properly into account the behavior
at small frequencies when fitting a given SD.
