  We show that electrodynamic dipolar interactions, responsible for long-range
fluctuations in matter, play a significant role in the stability of molecular
crystals. Density functional theory calculations with van der Waals
interactions determined from a semilocal "atom-in-a-molecule" model result in a
large overestimation of the dielectric constants and sublimation enthalpies for
polyacene crystals from naphthalene to pentacene, whereas an accurate treatment
of non-local electrodynamic response leads to an agreement with the measured
values for both quantities. Our findings suggest that collective response
effects play a substantial role not only for optical excitations, but also for
cohesive properties of non-covalently bound molecular crystals.
