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arXiv:0909.4942 · The Journal of Chemical Physics

Comment on “Equivalence of two approaches for quantum-classical hybrid systems” [J. Chem. Phys. 128, 204104 (2008)]

V. I. Gerasimenko

Published Sep 28, 2009• 1 citation
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Abstract

We discuss the links between consistent and approximate descriptions of the quantum-classical systems, i.e., systems composed of two interacting subsystems, one of which behaves almost classically while the other requires a quantum description.
DOI
10.1063/1.3240753

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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 H2_{2}2​ molecules using optimally shallow unitary circuits. We find that molecular energies remain size-consistent within chemical accuracy for an estimated 118 and 71 H2_{2}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.