# An analysis of the performance of coupled cluster methods for core excitations and core ionizations using standard basis sets

## TL;DR

This study evaluates the accuracy of various coupled cluster methods and basis sets in calculating core excitation and ionization energies for molecules, highlighting the significance of relativistic effects.

## Contribution

It provides a comprehensive comparison of coupled cluster methods and basis sets for core-level calculations, including relativistic effects, across multiple molecules.

## Key findings

- CC3 and CCSDT yield more accurate core excitation energies.
- Basis set extrapolation improves the estimation of complete basis set limits.
- Relativistic effects significantly influence core excitation and ionization energies.

## Abstract

An extensive analysis has been carried out of the performance of standard families of basis sets with the hierarchy of coupled cluster methods CC2, CCSD, CC3 and CCSDT in computing selected Oxygen, Carbon and Nitrogen K-edge (vertical) core excitation and ionization energies within a core-valence separated scheme in the molecules water, ammonia, and carbon monoxide. Complete basis set limits for the excitation energies have been estimated via different basis set extrapolation schemes. The importance of scalar relativistic effects has been established within the spin-free exact two-component theory in its one-electron variant (SFX2C-1e).