The difference I - A between the ionization potential and electron affinity of a molecule is one of the input parameters necessary for calculating the energies of charge transfer states in molecular crystals. In the present paper, the performance of the DFT methodology with several alternative forms of the exchange-correlation potential is tested in this context. Calculations are carried out for polyacenes to select the best potentials and to set up a scheme for correcting systematic errors of the approach, which is subsequently used to calculate the relevant quantity for sexithiophene and perylenetetracarboxylic dianhydride (PTCDA). Combined with independent calculations of the electrostatic energy of charge pairs in these crystals, the results are confronted with CT state energies observed in electro-absorption spectroscopy.