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

Juliane H. Fuglsbjerg; Peter Reinholdt; Erik Kjellgren; Phillip W. K. Jensen; Sonia Coriani; Jacob Kongsted; Stephan P. A. Sauer

arXiv · 2511.09730

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

Juliane H. Fuglsbjerg·Peter Reinholdt·Erik Kjellgren·Phillip W. K. Jensen·Sonia Coriani·Jacob Kongsted·Stephan P. A. Sauer

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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.

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Juliane H. Fuglsbjerg, Peter Reinholdt, Erik Kjellgren, Phillip W. K. Jensen, Sonia Coriani, Jacob Kongsted, Stephan P. A. Sauer. 2025-11-12. Orbital-Optimized Unitary Coupled Cluster for Indirect Nuclear Spin-Spin Coupling Constants within a Quantum Linear Response Framework. https://arxiv.org/abs/2511.09730

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