Approaching multiplet splitting in X-ray photoelectron spectra by density functional theory methods: NO and O2 molecules as examples

Abstract

The ability of density functional theory (DFT) based methods to predict the multiplet splitting arising from the core hole ionization of molecules such as NO and O2, exhibiting an open shell grounds state, is explored. In the NO molecule, N(1s) or O(1s) ionization leads to 3Π and 1Π multiplets whereas for O2, the presence of an O(1s) core hole leads to doublet and quartet multiplets with distinct BEs. Multiplet splittings obtained using different exchange-correlation functionals show an overall good agreement with experiment and minor variations within the functionals studied when spin contamination resulting from unrestricted DFT calculations is accounted for.