Computing the molecular ground state energy in a restricted active space using quantum annealing

TL;DR

This paper demonstrates improved quantum annealing techniques for calculating the ground-state energy of water molecules, achieving higher accuracy and larger problem sizes than previous methods, advancing practical quantum chemistry applications.

Contribution

It introduces enhanced annealing strategies and hardware utilization to solve larger molecular problems with higher accuracy using quantum annealing.

Findings

Achieved over double the success probability for Hartree-Fock solutions.

Extended problem size to nearly 2.5 times larger than previous reports.

Reduced energy difference to 0.120 Hartree relative to Hartree-Fock.

Abstract

Calculating the molecular ground-state energy is a central challenge in computational chemistry. Conventional methods such as the Complete Active Space Configuration Interaction scale exponentially with molecular size, limiting their applicability to large molecules. Quantum computing offers a promising alternative by mapping molecular Hamiltonians by qubits, enabling cheaper computational scaling. Previous studies have shown that it is possible to formulate molecular ground state calculations as discrete optimization problems, addressable by quantum annealing. However, these efforts have been limited by previous generations of hardware and suboptimal annealing techniques. Here, the $H_{2}O$ ground-state problem is mapped to an Ising Hamiltonian using the Xian-Bias-Kas (XBK) method. By taking advantage of enhanced qubit connectivity and shorter embedding chains, it is solved with a more than doubled probability of achieving Hartree-Fock-level solutions with respect to the most advanced predecessor. Advanced annealing strategies extend Hartree-Fock-level accuracy to significantly larger problem instances, enabling solutions that use nearly 2.5 times more physically embedded qubits than the largest cases previously reported and allowing to improve annealing results by two orders of magnitude, reaching an energy difference of 0.120~Hartree relative to Hartree-Fock. These results show tangible progress toward practical quantum annealing applications in NISQ era.