In-medium similarity renormalization group for a pairing-plus-particle-hole model

TL;DR

This paper compares two implementations of the IMSRG method, demonstrating that IMSRG(2*) provides more accurate results for low-lying states in a pairing-plus-particle-hole model, especially in strongly correlated systems.

Contribution

The study benchmarks IMSRG(2) and IMSRG(2*) implementations, highlighting the improved accuracy of IMSRG(2*) by including higher-body contributions.

Findings

IMSRG(2*) outperforms IMSRG(2) in accuracy

Convergence is more challenging for excited states in strongly correlated systems

IMSRG(2*) better captures higher-body effects

Abstract

We benchmark two implementations of the in-medium similarity renormalization group (IMSRG) method, IMSRG(2) and IMSRG(2*), for the low-lying states of a pairing-plus-particle-hole model with varying numbers of fermions. In IMSRG(2), all operators are truncated up to the normal-ordered two-body terms, whereas IMSRG(2*) includes an additional term to partially account for higher-body contributions. The results are compared against exact solutions. We find that IMSRG(2*) consistently outperforms IMSRG(2) for both ground and excited states, although achieving convergence for excited states remains more challenging in strongly correlated systems than for the ground state.