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This article belongs to the Special Issue 50th Anniversary of the Kohn-Sham Theory—Advances in Density Functional Theory., Ionization potentials (IPs) and electron affinities (EAs) are important quantities input into most models for calculating the open-circuit voltage (Voc) of organic solar cells. We assess the semi-empirical density-functional tight-binding (DFTB) method with the third-order self-consistent charge (SCC) correction and the 3ob parameter set (the third-order DFTB (DFTB3) organic and biochemistry parameter set) against experiments (for smaller molecules) and against first-principles GW (Green’s function, G, times the screened potential, W) calculations (for larger molecules of interest in organic electronics) for the calculation of IPs and EAs. Since GW calculations are relatively new for molecules of this size, we have also taken care to validate these calculations against experiments. As expected, DFTB is found to behave very much like density-functional theory (DFT), but with some loss of accuracy in predicting IPs and EAs. For small molecules, the best results were found with ΔSCF (Δ self-consistent field) SCC-DFTB calculations for first IPs (good to ± 0.649 eV). When considering several IPs of the same molecule, it is convenient to use the negative of the orbital energies (which we refer to as Koopmans’ theorem (KT) IPs) as an indication of trends. Linear regression analysis shows that KT SCC-DFTB IPs are nearly as accurate as ΔSCF SCC-DFTB eigenvalues (± 0.852 eV for first IPs, but ± 0.706 eV for all of the IPs considered here) for small molecules. For larger molecules, SCC-DFTB was also the ideal choice with IP/EA errors of ± 0.489/0.740 eV from ΔSCF calculations and of ± 0.326/0.458 eV from (KT) orbital energies. Interestingly, the linear least squares fit for the KT IPs of the larger molecules also proves to have good predictive value for the lower energy KT IPs of smaller molecules, with significant deviations appearing only for IPs of 15–20 eV or larger. We believe that this quantitative analysis of errors in SCC-DFTB IPs and EAs may be of interest to other researchers interested in DFTB investigation of large and complex problems, such as those encountered in organic electronics., This work is supported, in part, by the ORGAVOLT (ORGAnic solar cell VOLTage by numerical computation) Grant ANR-12-MONU-0014-02 of the French Agence Nationale de la Recherche (ANR) 2012 Programme Modèles Numériques. Mathias P. Ljungberg, Peter Koval and Daniel Sánchez-Portal acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) through the SFB (Sonderforschungsbereich) 1083 project, the Spanish MINECO (Ministerio de Economía y Competitividad) MAT2013-46593-C6-2-P project and the Euroregion Aquitaine-Euskadi program and support from the Basque Departamento de Educación, UPV/EHU (University of the Basque Country) (Grant No. IT-756-13). Peter Koval acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union. Ala Aldin M. H. M. Darghouth would like to thank the French and Iraq governments for support through a Campus France scholarship. Walid Taouali would like to acknowledge support from the government of Tunisia.