  (Semi)-local density functional approximations (DFAs) suffer from
self-interaction error (SIE). When the first ionization energy (IE) is computed
as the negative of the highest-occupied orbital (HO) eigenvalue, DFAs
notoriously underestimate them compared to quasi-particle calculations. The
inaccuracy for the HO is attributed to SIE inherent in DFAs. We assessed the IE
based on Perdew-Zunger self-interaction corrections on 14 small to
moderate-sized organic molecules relevant in organic electronics and polymer
donor materials. Though self-interaction corrected DFAs were found to
significantly improve the IE relative to the uncorrected DFAs, they
overestimate. However, when the self-interaction correction is interiorly
scaled using a function of the iso-orbital indicator z{\sigma}, only the
regions where SIE is significant get a correction. We discuss these approaches
and show how these methods significantly improve the description of the HO
eigenvalue for the organic molecules.
