The example of sexithiophene is used to illustrate some problems related to theoretical interpretation of electro-absorption spectra. Two different theoretical approaches are combined to calculate the energies of localized ionic pairs (charge-transfer states) in the sexithiophene crystal. In the lack of the corresponding experimental data, the ionization potential and electron affinity of the molecule are obtained from quantum chemical DFT-type calculations. The electrostatic energies in the crystal are provided by the microelectrostatic Fourier-transform approach. The probable accuracy of the results is estimated by testing their sensitivity to changes in the input data. Some related theoretical problems that have to be addressed in the future are pointed out.