  Due to recent advances in scanning-probe technology, the electronic structure
of individual molecules can now also be investigated if they are immobilized by
adsorption on non-conductive substrates. As a consequence, different molecular
charge-states are now experimentally accessible. Thus motivated, we investigate
as an experimentally relevant example the electronic and structural properties
of a NaCl(001) surface with and without pentacene adsorbed (neutral and
charged) by employing density functional theory. We estimate the polaronic
reorganization energy to be $E_\text{reorg} \simeq 0.8-1.0$ eV, consistent with
experimental results obtained for molecules of similar size. To account for
environmental effects on this estimate, different models for charge screening
are compared. Finally, we calculate the density profile of one of the frontier
orbitals for different occupation and confirm the experimentally observed
localization of the charge density upon relaxation of the substrate from
ab-initio calculations.
