The application of periodic boundary conditions-density functional theory (PBC-DFT) to a number of donor-acceptor (D-A) copolymers used in photovoltaic devices is investigated and compared to traditional Kuhn fits and experimental data. The rbital energies were calculated by means of the hybrid density functional B3LYP. The band gaps from the oligomer approach are 0.11 eV lower than those of the PBC method. Statistical correlation for the PBC calculations at the higher basis set with experimental data show a slight advantage for using the higher basis set to correlate calculated and experimental data for this series of D-A copolymers. Experimental data suggest substitution of 2,1,3-benzothiadiazole with 2,1,3-benzoselenadiazole in a copolymers with constant donor groups affords a reduced band gap. Calculated HOCO levels are consistent with the experimental HOMO trends for these subsets.