XABOOM: An X‑ray Absorption Benchmark of Organic Molecules Based on Carbon, Nitrogen, and Oxygen 1s → π* Transitions

The performance of
several standard and popular approaches for
calculating X-ray absorption spectra at the carbon, nitrogen, and
oxygen K-edges of 40 primarily organic molecules up to the size of
guanine has been evaluated, focusing on the low-energy and intense
1s → π* transitions. Using results obtained with CVS-ADC(2)-x
and fc-CVS-EOM-CCSD as benchmark references, we investigate the performance
of CC2, ADC(2), ADC­(3/2), and commonly adopted density functional
theory (DFT)-based approaches. Here, focus is on precision rather than on accuracy of transition energies
and intensitiesin other words, we target relative energies
and intensities and the spread thereof, rather than absolute values.
The use of exchange–correlation functionals tailored for time-dependent
DFT calculations of core excitations leads to error spreads similar
to those seen for more standard functionals, despite yielding superior
absolute energies. Long-range corrected functionals are shown to perform
particularly well compared to our reference data, showing error spreads
in energy and intensity of 0.2–0.3 eV and ∼10%, respectively,
as compared to 0.3–0.6 eV and ∼20% for a typical pure
hybrid. In comparing intensities, state mixing can complicate matters,
and techniques to avoid this issue are discussed. Furthermore, the
influence of basis sets in high-level ab initio calculations
is investigated, showing that reasonably accurate results are obtained
with the use of 6-311++G**. We name this benchmark suite as XABOOM
(X-ray absorption benchmark of organic molecules) and provide molecular
structures and ground-state self-consistent field energies and spectroscopic
data. We believe that it provides a good assessment of electronic
structure theory methods for calculating X-ray absorption spectra
and will become useful for future developments in this field.