# Open-Shell Nuclei from No-Core Shell Model with Perturbative Improvement

## TL;DR

This paper presents a hybrid approach combining No-Core Shell Model and Multi-Configurational Perturbation Theory to efficiently compute energies of open-shell nuclei, achieving accuracy comparable to more expensive methods.

## Contribution

It introduces a novel hybrid method that improves computational efficiency for open-shell nuclei by integrating NCSM with MCPT corrections.

## Key findings

- Accurately computes ground and excited states of various isotopes.
- Achieves results comparable to large-scale NCSM calculations.
- Reduces computational cost significantly.

## Abstract

We introduce a hybrid many-body approach that combines the flexibility of the No-Core Shell Model (NCSM) with the efficiency of Multi-Configurational Perturbation Theory (MCPT) to compute ground- and excited-state energies in arbitrary open-shell nuclei in large model spaces. The NCSM in small model spaces is used to define a multi-determinantal reference state that contains the most important multi-particle multi-hole correlations and a subsequent second-order MCPT correction is used to capture additional correlation effects from a large model space. We apply this new ab initio approach for the calculation of ground-state and excitation energies of even and odd-mass carbon, oxygen, and fluorine isotopes and compare to large-scale NCSM calculations that are computationally much more expensive.