A benchmark study of core-excited states of organic molecules computed with the generalized active space driven similarity renormalization group

TL;DR

This study evaluates the accuracy of the GAS-DSRG method for computing X-ray absorption spectra of organic molecules, demonstrating that third-order corrections significantly improve energy predictions.

Contribution

It introduces a benchmark analysis of GAS-DSRG for core-excited states, highlighting the importance of higher-order perturbation corrections for accuracy.

Findings

Third-order perturbation improves excitation energy accuracy.

GAS-DSRG systematically underestimates energies at second order.

Active space truncation and intruder states affect results.

Abstract

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.