Screened Coulomb interaction in dielectrics is often used as an argument for a lower exciton binding energy and easier exciton dissociation in a high dielectric material. In this paper, we show that at length scales of excitons (10-20 angstrom), the screened Coulomb law is invalid and a microscopic (quantum chemical) description is necessary to describe the medium effect on exciton dissociation. The exciton dissociation energy decreases with increasing dielectric constant, albeit deviating from the inversely proportional relationship. The electron-hole interaction energy, approximated with a point charge model, is apparently not affected by the dielectric constant of the environment. (C) 2014 Elsevier B.V. All rights reserved.