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.

Figure 1

(Color online) Simulated and measured photoemission spectra of PTCDA. From top to bottom: isosurface plots of Kohn-Sham orbitals (labels HOMO, HOMO-1, etc., refer to the LDA ordering), Kohn-Sham eigenvalues broadened by 0.1 eV for LDA and GKLI, and gas phase experimental data from Ref. 9. Note that the experiment shows a pronounced gap between the HOMO and the HOMO-1 peaks that is reproduced by the GKLI spectrum but not by the LDA one.

Figure 2

(Color online) Orbital self-interaction $e_i$ for the least bound LDA orbitals of PTCDA, NTCDA, pentacene, and ${\text{Si}}_4{\text{D}}^{-}$ relative to the respective $e_{\text{HOMO}}$. Dashed lines are a guide to the eye.

Figure 3

(Color online) DOS plots for NTCDA from occupied Kohn-Sham eigenvalues as obtained in LDA and GKLI, and isosurface plots of Kohn-Sham orbitals as in Fig. 1. Again, GKLI opens a gap between HOMO and HOMO-1 that is not seen with the LDA.

Figure 4

(Color online) Orbital shift relative to the shift for the HOMO as obtained from the GKLI calculation (blue triangles) and Eq. (2) (black circles) for PTCDA (top) and NTCDA (bottom). The HOMO is orbital number 70 (top) or 48 (bottom), respectively.