Alternative similarity renormalization group generators in nuclear structure calculations

TL;DR

This paper explores alternative similarity renormalization group (SRG) generators in nuclear physics to reduce the induction of many-body forces while maintaining fast convergence in ab initio calculations.

Contribution

It introduces and tests two new SRG generators with block-structured operators, showing promising results in no-core shell model calculations.

Findings

Alternative generators reduce induced many-body forces.

New generators maintain good convergence in calculations.

Promising performance in light nuclei calculations.

Abstract

The similarity renormalization group (SRG) has been successfully applied to soften interactions for ab initio nuclear calculations. In almost all practical applications in nuclear physics, an SRG generator with the kinetic energy operator is used. With this choice, a fast convergence of many-body calculations can be achieved, but at the same time substantial three-body interactions are induced even if one starts from a purely two-nucleon (NN) Hamiltonian. Three-nucleon (3N) interactions can be handled by modern many-body methods. However, it has been observed that when including initial chiral 3N forces in the Hamiltonian, the SRG transformations induce a non-negligible four-nucleon interaction that cannot be currently included in the calculations for technical reasons. Consequently, it is essential to investigate alternative SRG generators that might suppress the induction of many-body forces while at the same time might preserve the good convergence. In this work we test two alternative generators with operators of block structure in the harmonic oscillator basis. In the no-core shell model calculations for 3H, 4He and 6Li with chiral NN force, we demonstrate that their performances appear quite promising.