Natural orbitals for ab initio no-core shell model calculations

TL;DR

This paper demonstrates that natural orbitals derived from a correlated density matrix significantly improve the convergence and robustness of ab initio no-core shell model calculations, outperforming traditional bases like Hartree-Fock and harmonic oscillator.

Contribution

It introduces a novel basis set based on natural orbitals from a correlated density matrix, enhancing convergence efficiency in no-core shell model calculations.

Findings

Natural orbitals outperform Hartree-Fock and harmonic oscillator bases.

Faster and more robust convergence with natural orbitals.

Effective for various observables in p-shell nuclei.

Abstract

We explore the impact of optimizations of the single-particle basis on the convergence behavior and robustness of ab initio no-core shell model calculations. Our focus is on novel basis sets defined by the natural orbitals of a correlated one-body density matrix that is obtained in second-order many-body perturbation theory. Using a perturbatively improved density matrix as starting point informs the single-particle basis about the dominant correlation effects on the global structure of the many-body state, while keeping the computational cost for the basis optimization at a minimum. Already the comparison of the radial single-particle wavefunctions reveals the superiority of the natural-orbital basis compared to a Hartree-Fock or harmonic oscillator basis, and it highlights pathologies of the Hartree-Fock basis. We compare the model-space convergence of energies, root-mean-square radii, and selected electromagnetic observables with all three basis sets for selected p-shell nuclei using chiral interactions with explicit three-nucleon terms. In all cases the natural-orbital basis provides the fastest and most robust convergence, making it the most efficient basis for no-core shell model calculations. As an application we present no-core shell model calculations for the ground-state energies of all oxygen isotopes and assess the accuracy of the normal-ordered two-body approximation of the three-nucleon interaction in the natural-orbital basis.