# Non-Empirical Interactions for the Nuclear Shell Model: An Update

## TL;DR

This paper reviews recent advances in ab initio methods for the nuclear shell model, emphasizing the in-medium similarity renormalization group and the importance of three-body forces, while comparing theoretical results with experimental data.

## Contribution

It provides an update on ab initio shell model methods, analyzes their connections, and discusses the role of three-body forces in improving phenomenological models.

## Key findings

- Three-body forces are crucial for accurate shell model descriptions.
- The in-medium similarity renormalization group offers promising insights.
- Comparisons with experiments highlight remaining challenges.

## Abstract

The nuclear shell model has been perhaps the most important conceptual and computational paradigm for the understanding of the structure of atomic nuclei. While the shell model has been predominantly used in a phenomenological context, there have been efforts stretching back over a half century to derive shell model parameters based on a realistic interaction between nucleons. More recently, several ab initio many-body methods---in particular many-body perturbation theory, the no-core shell model, the in-medium similarity renormalization group, and coupled cluster theory---have developed the capability to provide effective shell model Hamiltonians. We provide an update on the status of these methods and investigate the connections between them and potential strengths and weaknesses, with a particular focus on the in-medium similarity renormalization group approach. Three-body forces are demonstrated to be an important ingredient in understanding the modifications needed in phenomenological treatments. We then review some applications of these methods to comparisons with recent experimental measurements, and conclude with some remaining challenges in ab initio shell model theory.