Accurate computation of X-ray absorption spectra with ionization potential optimized global hybrid functional.

This manuscript presents the first consistent ionization potential (IP) optimized global hybrid functional to accurately estimate the vertical ionization and excitation energies of the inner-shell electrons in molecules. The new method fulfills the IP theorem that the Kohn-Sham eigenvalues of all the occupied orbitals (including the core orbitals) are good approximations to the exact vertical ionization energies. The accuracy of the one-particle spectrum is essential to enabling the one-particle density functional theory (DFT) to provide accurate results. Compared to its precursor, the range-separated hybrid functional CAM-QTP00, the new method is more computationally efficient. The IP theorem enables the new method to provide inner-shell ionization energies measured by X-ray photoelectron spectroscopy, and it can further accurately simulate the X-ray absorption spectrum (XAS, or NEXAFS). The simulated spectra can be compared to the experiment directly without shifting. In addition, the new method reduces the delocalization error (many-electron self-interaction error) which is a severe problem in DFT.