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

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, CCSD, and CC3 results. qLR with UCC/ooUCC yields spin-spin coupling constants comparable to those of classical methods. We further examine the role of orbital optimization and find that ooUCC markedly affects the computed couplings; the orbital-optimized results show better agreement with CCSD and CC3. These findings indicate that orbital response is important for accurate NMR coupling predictions within quantum-computing-friendly correlated methods.