Acene-thiophenes compounds have been used successfully as active materials in thin-film transistors and photodetectors. This work aims at obtaining an adequate theoretical framework to accurately characterize the optical and electronic properties of two such compounds: NaT2 and NaT3. This is done by comparing the results of simulations with experimental absorption spectra. Basis size effects are investigated as well as the role of intramolecular vibrations in the simulated spectra. It is shown that the most important feature of a DFT calculation is the appropriate selection of long-range corrected functionals, which allows for the accurate description of the first absorption band of these molecules.