  We present a novel hybrid quantum/classical (QM/MM) approach to the
calculation of charged excitations in molecular solids based on the many-body
Green's function $GW$ formalism. Molecules described at the $GW$ level are
embedded into the crystalline environment modeled with an accurate classical
polarizable scheme. This allows the calculation of electron addition and
removal energies in the bulk and at crystal surfaces where charged excitations
are probed in photoelectron experiments. By considering the paradigmatic case
of pentacene and perfluoropentacene crystals, we discuss the different
contributions from intermolecular interactions to electronic energy levels,
distinguishing between polarization, which is accounted for combining quantum
and classical polarizabilities, and crystal field effects, that can impact
energy levels by up to $\pm0.6$ eV. After introducing band dispersion, we
achieve quantitative agreement (within 0.2 eV) on the ionization potential and
electron affinity measured at pentacene and perfluoropentacene crystal surfaces
characterized by standing molecules.
