  The structural and electronic properties of a single pentacene molecule and a
pentacene molecular crystal, an organic semiconductor, are examined by a
first-principles method based on the generalized gradient approximation of
density functional theory. Calculations were carried out for a triclinic unit
cell containing two pentacene molecules. The bandwidths of the valence and
conduction bands which determine the charge migration mechanism are found to
depend strongly on the crystallographic direction. Along the triclinic
reciprocal lattice vectors A and B which are orientated approximately
perpendicular to the molecular axes the maximal valence (conduction) band width
amounts to only 75 (59) meV, even smaller values are obtained for the C
direction parallel to molecular axes even less. Along the stacking directions
A+B and A-B, however, the maximal valence (conduction) band width is found to
reach 145 (260) meV. The value for the conduction band width is larger than
estimates for the polaron binding energy but significantly smaller than recent
results obtained by semiempirical methods. The single molecule has a HOMO-LUMO
gap of about 1.1 eV as deduced from the Kohn-Sham eigenvalue differences. When
using the self-consistent field method, which is expected to yield more
reliable results, a value of 1.64 eV is obtained. The theoretical value for the
band gap in the molecular solid amounts to 1.0 eV at the Gamma-point.
