posted on 2024-01-04, 15:09authored byDiptarka Hait, Todd J. Martínez
X-ray absorption spectra (XAS) of biradicaloid species
are often
thought to represent a challenge to theoretical methods. This has
led to the 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 individual CSFs, despite the presence of
multireference character in the ground state. We also report that
the 1s → π* and 1s → σ* transitions have
quite distinct widths for O3. 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 experiments instead of the popular procedure of uniformly
broadening stick spectra at the equilibrium geometry.