Charge-carrier transport in thin-film organic field-effect transistors takes place within the first (few) molecular layer(s) of the active organic material in contact with the gate dielectric. Here, we use atomistic molecular dynamics simulations to evaluate how interactions with bare amorphous silica surfaces that vary in terms of surface potential influence the molecular packing and dynamics of a monolayer pentacene film. The results indicate that the long axis of the pentacene molecules has a non-negligible tilt angle away from the surface normal. Grazing-incidence X-ray diffraction patterns for these models are calculated, and we discuss notable differences in the shapes of the Bragg rods as a function of the molecular packing, also in relation to previously published experimental reports. Intermolecular electronic couplings (transfer integrals) evaluated for the monolayers show marked differences compared to bulk crystal calculations, a result that points to the importance of fully considering the molecular packing environment in charge-carrier mobility models for organic electronic materials.