Size-Consistent Quantum Chemistry on Quantum Computers

TL;DR

This paper evaluates the size consistency of quantum chemistry algorithms on current quantum hardware, demonstrating that they maintain accuracy for systems with over a hundred molecules, supporting scalable quantum simulations.

Contribution

It systematically assesses size consistency on quantum devices, showing that current hardware preserves this property for large molecular systems, enabling scalable quantum chemistry.

Findings

Size consistency is maintained for up to 118 H₂ molecules with one-qubit circuits.

Size consistency is maintained for up to 71 H₂ molecules with two-qubit circuits.

Current quantum devices support scalable, noise-resilient molecular simulations.

Abstract

Hybrid quantum-classical algorithms have begun to leverage quantum devices to efficiently represent many-electron wavefunctions, enabling early demonstrations of molecular simulations on real hardware. A key prerequisite for scalable quantum chemistry, however, is size consistency: the energy of non-interacting subsystems must scale linearly with system size. While many algorithms are theoretically size-consistent, noise on quantum devices may couple nominally independent subsystems and degrade this fundamental property. Here, we systematically evaluate size consistency on quantum hardware by simulating systems composed of increasing numbers of non-interacting H$_{2}$ molecules using optimally shallow unitary circuits. We find that molecular energies remain size-consistent within chemical accuracy for an estimated 118 and 71 H$_{2}$ subsystems for one- and two-qubit unitary designs, respectively, demonstrating that current quantum devices preserve size consistency over chemically relevant system sizes and supporting the feasibility of scalable, noise-resilient simulation of strongly correlated molecules and materials.