  The uniaxial negative thermal expansion in pentacene crystals along $a$ is a
particularity in the series of the oligoacenes, and exeptionally large for a
crystalline solid. Full x-ray structure analysis from 120 K to 413 K reveals
that the dominant thermal motion is a libration of the rigid molecules about
their long axes, modifying the intermolecular angle which describes the
herringbone packing within the layers. This herringbone angle increases with
temperature (by 0.3 -- 0.6$^{\circ}$ per 100 K), and causes an anisotropic
rearrangement of the molecules within the layers, i.e. an expansion in the $b$
direction, and a distinct contraction along $a$. Additionally, a larger
herringbone angle improves the cofacial overlap between adjacent, parallel
molecules, and thus enhances the attractive van der Waals forces.
