When to trust photoelectron spectra from Kohn-Sham eigenvalues: The case of organic semiconductors

The combination of photoelectron spectroscopy and density functional theory is an important technique for clarifying a material's electronic structure. So far, however, it has been difficult to predict when the spectrum of occupied Kohn-Sham eigenvalues obtained from commonly used (semi-)local functionals bears physical relevance and when not. We demonstrate that a simple criterion based on evaluating each orbital's self-interaction allows prediction of the physical reliability of the eigenvalue spectrum. We further show that a self-interaction correction significantly improves the interpretability of eigenvalues also in difficult cases such as organic semiconductors where (semi-)local functionals fail.