Predicting the X-ray Absorption Spectrum of Ozone with Single Configuration State Functions.

X-ray absorption spectra (XAS) of biradicaloid species are often thought to represent a challenge to theoretical methods. This has led to testing of recently developed multireference techniques on the XAS of ozone, but reproduction of the experimental spectral profile has proven difficult. We utilize a minimal model consisting of a single configuration state function (CSF) per excited state to model core-level excitations of ozone, with the orbitals of each CSF optimized using the restricted open-shell Kohn-Sham (ROKS) method. This protocol leads to semiquantitative agreement with experimental XAS. In fact, we find that low-lying core-hole excited states in biradicaloids can be approximated with single CSFs despite ground state multireference character. We also report that the 1s→π^* and 1s→σ^* transitions have quite distinct widths for O_3. This reveals the importance of sampling over a representative range of geometries from the vibrational ground state for properly assessing the accuracy of electronic structure methods against experiment, instead of the popular procedure of uniformly broadening stick spectra at the equilibrium geometry.