  We investigate on the procedure of extracting a "spectral density" from mixed
QM/MM calculations and employing it in open quantum systems models. In
particular, we study the connection between the energy gap correlation function
extracted from ground state QM/MM and the bath spectral density used as input
in open quantum system approaches. We introduce a simple model which can give
intuition on when the ground state QM/MM propagation will give the correct
energy gap. We also discuss the role of higher order correlators of the
energy-gap fluctuations which can provide useful information on the bath.
Further, various semiclassical corrections to the spectral density, are applied
and investigated. Finally, we apply our considerations to the photosynthetic
Fenna-Matthews-Olson complex. For this system, our results suggest the use of
the Harmonic prefactor for the spectral density rather than the Standard one,
which was employed in the simulations of the system carried out to date.
