  We report on electronic-structure calculations for the pentacene and rubrene
crystals, based on experimental crystal geometries measured at different
temperatures. The results are in very good agreement with angle-resolved
photoelectron spectroscopy data that indicate that the widths of the valence
and conduction bands in both materials become narrower at higher temperatures.
Our findings strongly suggest that the thermal bandwidth narrowing in the
pentacene and rubrene crystals is primarily caused by the thermal expansion of
the lattice rather than by a renormalization of the transfer integrals induced
by a polaron effect. The effect of thermal expansion on the charge-transport
properties is also discussed.
