Orbital-Optimized Unitary Coupled Cluster for Indirect Nuclear Spin-Spin Coupling Constants within a Quantum Linear Response Framework

TL;DR

This paper introduces a quantum linear response method using orbital-optimized unitary coupled cluster to accurately compute nuclear spin-spin coupling constants, demonstrating its effectiveness compared to classical approaches.

Contribution

It develops a quantum-compatible orbital-optimized UCC approach within a linear response framework for NMR coupling calculations, highlighting the importance of orbital optimization.

Findings

qLR with UCC/ooUCC produces results comparable to classical methods

Orbital optimization significantly improves coupling constant accuracy

The method is suitable for quantum computing implementations

Abstract

We present a quantum linear response (qLR) approach within an active-space framework for computing indirect nuclear spin-spin coupling constants, a key ingredient in NMR spectra predictions. The method employs the unitary coupled cluster (UCC) ansatz and its orbital-optimized variant (ooUCC), both suitable for quantum computing implementations, to evaluate spin-spin coupling constants via qLR. Test calculations on five small molecules are compared with CASCI, CASSCF, and conventional CCSD results. qLR with UCC/ooUCC yields spin-spin coupling constants comparable to classical methods. We further examine the role of orbital optimization and find that ooUCC markedly affects the computed couplings; orbital-optimized results show better agreement with CCSD. These findings indicate that orbital optimization is important for accurate NMR coupling predictions within quantum-computing-friendly correlated methods.