Ab Initio Description of Open-Shell Nuclei: Merging No-Core Shell Model and In-Medium Similarity Renormalization Group

TL;DR

This paper introduces a novel combined ab initio method merging NCSM and IM-SRG to enable comprehensive and efficient calculations of ground and excited states across a wide range of nuclei, including medium-mass isotopes.

Contribution

The paper presents a new many-body approach that integrates NCSM and IM-SRG, allowing fully converged no-core calculations for diverse nuclei with moderate computational effort.

Findings

Successfully applied to carbon and oxygen isotopic chains.

Achieves rapid convergence and broad applicability.

Enables detailed spectroscopy of medium-mass nuclei.

Abstract

We merge two successful ab initio nuclear-structure methods, the no-core shell model (NCSM) and the multi-reference in-medium similarity renormalization group (IM-SRG) to define a new many-body approach for the comprehensive description of ground and excited states of closed and open-shell nuclei. Building on the key advantages of the two methods---the decoupling of excitations at the many-body level in the IM-SRG and the access to arbitrary nuclei, eigenstates, and observables in the NCSM---their combination enables fully converged no-core calculations for an unprecedented range of nuclei and observables at moderate computational cost. We present applications in the carbon and oxygen isotopic chains, where conventional NCSM calculations are still feasible and provide an important benchmark. The efficiency and rapid convergence of the new approach make it ideally suited for ab initio studies of the complete spectroscopy of nuclei up into the medium-mass regime.