Hyperfine Coupling Constants on Quantum Computers: Performance, Errors, and Future Prospects

We present the first implementation and computation of electron spin resonance isotropic hyperfine coupling constants (HFCs) on quantum hardware. As illustrative test cases, we compute the HFCs for the hydroxyl radical (OH$^{\bullet}$), nitric oxide (NO$^{\bullet}$), and the triplet hydroxyl cation (OH$^{+}$). Our approach integrates the qubit-ADAPT method with unrestricted orbital optimization in an active space framework. To accurately measure the necessary spin one-electron reduced density matrices on current hardware, we employ a combination of error mitigation, error suppression, and post-selection, including our in-house developed ansatz-based readout and gate error mitigation. The HFCs obtained from the quantum hardware experiments align with results from unrestricted complete active space self-consistent field calculations on classical hardware. These results mark a significant step towards leveraging quantum computing for chemically relevant molecular properties and highlight the critical role of multi-method error strategies in the noisy intermediate-scale quantum era.