Molecular dynamics simulations are presented for "bulklike" and "filmlike" monolayers of pentacene deposited on a slab of amorphous silica. The two simulated systems, which mainly differ in the tilt angle between the pentacene molecules and the silica surface, exhibit structural and energetic properties that match the available measurements. The bulklike monolayer, the structure of which corresponds to that of the low-temperature polymorph of crystalline pentacene, is stable. The filmlike monolayer, in which the molecules are most closely normal to the surface, is instead thermodynamically metastable, in agreement with the experimental evidence.