  This work examines the accuracy and precision of X-ray absorption spectra
computed with a multireference approach that combines generalized active space
(GAS) references with the driven similarity renormalization group (DSRG). We
employ the X-ray absorption benchmark of organic molecules (XABOOM) set,
consisting of 116 transitions from mostly organic molecules [T. Fransson et
al., J. Chem. Theory Comput. 17, 1618 (2021)]. Several approximations to a
full-valence active space are examined and benchmarked. Absolute excitation
energies and intensities computed with the GAS-DSRG truncated to second-order
in perturbation theory are found to systematically underestimate experimental
and reference theoretical values. Third-order perturbative corrections
significantly improve the accuracy of GAS-DSRG absolute excitation energies,
bringing the mean absolute deviation from experimental values down to 0.32 eV.
The ozone molecule and glyoxylic acid are particularly challenging for
second-order perturbation theory and are examined in detail to assess the
importance of active space truncation and intruder states.
