Open-shell nuclei from No-Core Shell Model with perturbative improvement

Alexander Tichai,Eskendr Gebrerufael,Klaus Vobig,Robert Roth

doi:10.1016/j.physletb.2018.10.029

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.

Highlights

The solution of the nuclear many-body problem with realistic interactions is at the heart of ab initio nuclear structure theory
The ground state of these nuclei is dominated by a single Slater determinant that can serve as a reference state for the construction of the fully correlated eigenstate
Isotopes in the vicinity of shell closures can be tackled by equation-ofmotion techniques build on the ground state of a neighbouring closed-shell nucleus [4]

Summary

Introduction

The solution of the nuclear many-body problem with realistic interactions is at the heart of ab initio nuclear structure theory. Using second-order perturbative corrections we perform a detailed study of ground-state and excitation energies for carbon and oxygen isotopes and benchmark with large-scale NCSM calculations. The importance-truncated NCSM calculations within the NO2B approximation are performed up to Nmax = 10 using an optimized natural orbital single-particle basis obtained from diagonalizing a MBPT-corrected one-body density The use of such natural orbitals improves the modelspace convergence and eliminates the dependence on the underlying oscillator frequency [38]. In addition to the NCSM-PT results including the second-order correction for Nmreafx = 0 and 2, we show the reference energy, i.e., the NCSM eigenvalue obtained in the Nmreafx space For these calculations we use a Hartree-Fock single-particle basis in order to further optimize the reference states. We further note that absolute energies in NCSM are far from being converged

Conclusion and Outlook

NCSM-PT