  Molecular crystals are a prototypical class of van der Waals (vdW) bound
organic materials with excited state properties relevant for optoelectronics
applications. Predicting the structure and excited state properties of
molecular crystals presents a challenge for electronic structure theory, as
standard approximations to density functional theory (DFT) do not capture long
range vdW dispersion interactions and do not yield excited state properties. In
this work, we use a combination of DFT including vdW forces) using both non
local correlation functionals and pair wise correction methods (together with
many body perturbation theory (MBPT) to study the geometry and excited states,
respectively, of the entire series of oligoacene crystals, from benzene to
hexacene. We find that vdW methods can predict lattice constants within 1
percent of the experimental measurements, on par with the previously reported
accuracy of pairwise approximations for the same systems. We further find that
excitation energies are sensitive to geometry, but if optimized geometries are
used MBPT can yield excited state properties within a few tenths of an eV from
experiment. We elucidate trends in MBPT computed charged and neutral excitation
energies across the acene series and discuss the role of common approximations
used in MBPT.
