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 a quantum hardware. As illustrative test cases, we compute the HFCs for the hydroxyl radical (OH•), nitric oxide (NO•), and 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 postselection, 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 toward leveraging quantum computing for chemically relevant molecular properties and highlight the critical role of multimethod error strategies in the noisy intermediate-scale quantum era.