# Relativistic coupled-cluster theory analysis of unusually large correlation effects in the determination of $g_j$ factors in Ca$^+$

## TL;DR

This paper uses relativistic coupled-cluster theory to analyze electron correlation effects on the $g_j$ factors of Ca$^+$ ion states, revealing the importance of non-linear and higher excitations for accurate results.

## Contribution

It demonstrates the necessity of including non-linear and triple excitation terms in RCC calculations to accurately determine $g_j$ factors in Ca$^+$.

## Key findings

- Non-linear terms significantly affect $g_j$ values.
- Higher-level excitations are crucial for precision.
- Perturbative approaches are insufficient for accuracy.

## Abstract

We investigate roles of electron correlation effects in the determination of the $g_j$ factors of the $4s ~ ^2S_{1/2}$, $4p ~ ^2P_{1/2}$, $4p ~ ^2P_{3/2}$, $3d ~ ^2D_{3/2}$, and $3d ~ ^2D_{5/2}$ states, representing to different parities and angular momenta, of the Ca$^+$ ion. Correlation contributions are highlighted with respect to the mean-field values evaluated using the Dirac-Hartree-Fock method, relativistic second order many-body theory, and relativistic coupled-cluster (RCC) theory with the singles and doubles approximation considering only the linear terms and also accounting for all the non-linear terms. This shows that it is difficult to achieve reasonably accurate results employing an approximated perturbative approach. We also find that contributions through the non-linear terms and higher-level excitations such as triple excitations, estimated perturbatively in the RCC method, are found to be crucial to attain precise values of the $g_j$ factors in the considered states of Ca$^+$ ion.