# Simulating X-ray absorption spectra with linear-response density cumulant theory

## TL;DR

This paper introduces a new linear-response density cumulant theory method, CVS-ODC-12, for simulating X-ray absorption spectra, showing good agreement with experimental data and advantages in computational efficiency.

## Contribution

The paper develops and benchmarks CVS-ODC-12, a novel method combining LR-DCT with core-valence separation for efficient and accurate X-ray absorption spectra simulation.

## Key findings

- Good agreement with experimental spectra for transition spacings and intensities.
- Systematic overestimation of excitation energies.
- Comparable performance to coupled cluster methods in spectral features.

## Abstract

We present a new approach for simulating X-ray absorption spectra based on linear-response density cumulant theory (LR-DCT) [A. V. Copan and A. Yu. Sokolov, J. Chem. Theory Comput., 2018, 14, 4097 - 4108]. Our new method combines the LR-ODC-12 formulation of LR-DCT with core-valence separation approximation (CVS) that allows to efficiently access high-energy core-excited states. We describe our computer implementation of the CVS-approximated LR-ODC-12 method (CVS-ODC-12) and benchmark its performance by comparing simulated X-ray absorption spectra to those obtained from experiment for several small molecules. Our results demonstrate that the CVS-ODC-12 method shows a good agreement with experiment for relative spacings between transitions and their intensities, but the excitation energies are systematically overestimated. When comparing to results from excited-state coupled cluster methods with single and double excitations, the CVS-ODC-12 method shows a similar performance for intensities and peak separations, while coupled cluster spectra are less shifted, relative to experiment. An important advantage of CVS-ODC-12 is that its excitation energies are computed by diagonalizing a Hermitian matrix, which enables efficient computation of transition intensities.