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arxiv: 2512.18395 · v1 · submitted 2025-12-20 · 🪐 quant-ph · physics.chem-ph· physics.comp-ph

Size-Consistent Quantum Chemistry on Quantum Computers

Pith reviewed 2026-05-16 20:35 UTC · model grok-4.3

classification 🪐 quant-ph physics.chem-phphysics.comp-ph
keywords size consistencyquantum chemistryquantum computershybrid quantum-classical algorithmsmolecular simulationH2 moleculesunitary circuitsnoise effects
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The pith

Quantum hardware maintains size-consistent molecular energies for up to 118 non-interacting H2 molecules.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines whether simulations on quantum computers keep the energy of many separate molecules adding up correctly as the number of molecules grows. This property, called size consistency, is essential for any reliable method to handle large chemical systems. By running calculations on groups of non-interacting H2 molecules with special shallow circuits, the work shows that the energies stay accurate even for dozens of molecules. A sympathetic reader would care because it indicates that noise in real quantum devices does not yet destroy this basic scaling behavior at practical sizes.

Core claim

By simulating systems composed of increasing numbers of non-interacting H₂ molecules using optimally shallow unitary circuits on quantum hardware, molecular energies remain size-consistent within chemical accuracy for an estimated 118 and 71 H₂ subsystems for one- and two-qubit unitary designs, respectively.

What carries the argument

Optimally shallow unitary circuits used to represent the wavefunctions of each H2 molecule in hybrid quantum-classical algorithms.

If this is right

  • Molecular energies scale linearly with the number of non-interacting H2 subsystems on current quantum devices.
  • Size consistency holds within chemical accuracy for system sizes relevant to chemistry.
  • The approach supports scalable, noise-resilient simulation of strongly correlated molecules and materials.
  • Both one-qubit and two-qubit unitary designs preserve size consistency at these scales.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Tests on molecules larger than H2 could show whether the result extends to systems with stronger electron correlation.
  • Reduced hardware noise would likely push the limit of preserved size consistency to even more subsystems.
  • Modular assembly of large-molecule calculations from consistent small-unit results may become practical.

Load-bearing premise

Noise on the quantum hardware does not create unexpected interactions between separate H2 molecules that would break the linear scaling of total energy.

What would settle it

A measurement on the same quantum hardware showing that the energy per H2 molecule deviates from the isolated-molecule value by more than chemical accuracy once the number of subsystems reaches around 100.

Figures

Figures reproduced from arXiv: 2512.18395 by David A. Mazziotti, Michael Rose, Noah Garrett.

Figure 1
Figure 1. Figure 1: Energy per H2 as system size (N) increases. Energies are calculated with single-qubit (left) and two-qubit (right) representations per subsystem. The energies for the single-qubit representation are simulated using the selective sample procedure (N=2,4,8,16; n=8,4,2,1; k=3) with the exception of N=1, which is generated using the random sample procedure (s=50). The energies for the two-qubit representation … view at source ↗
Figure 2
Figure 2. Figure 2: (a) Average double-excitation population per H2 subsystem as system size in￾creases. (b) Average single-excitation population per H2 as system size increases. The CISD result overlaps exactly with the FCI line. The populations for both figures are represented as single-qubit: × (orange), two-qubit: + (blue), four-qubit: ◦ (green), CISD: − (purple), and FCI: − (black). 9 [PITH_FULL_IMAGE:figures/full_fig_p… view at source ↗
Figure 3
Figure 3. Figure 3: Average energy error per H2 subsystem as system size increases. The energy errors are represented as single-qubit: × (orange), two-qubit: + (blue), Hartree-Fock: · · · (red), and FCI: − (black). The average error per H2 for each set of energies is shown in [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper evaluates size consistency of hybrid quantum-classical simulations on quantum hardware by computing energies of increasing numbers of non-interacting H₂ molecules using optimally shallow one- and two-qubit unitary circuits. It reports that energies remain within chemical accuracy up to extrapolated limits of 118 and 71 H₂ subsystems, respectively, and concludes that current devices preserve size consistency at chemically relevant scales.

Significance. If the extrapolation and underlying noise model are validated, the result would provide concrete evidence that hardware noise does not destroy size consistency at scales relevant to molecular simulation, strengthening the case for scalable quantum chemistry on near-term devices. The work directly addresses a key prerequisite for practical quantum advantage in chemistry.

major comments (2)
  1. [Abstract] Abstract: the headline limits of 118 and 71 H₂ subsystems are extrapolations obtained by fitting energy deviations on small numbers of molecules to a noise model and projecting forward; the manuscript must detail the functional form of the fit, the exact range of system sizes actually executed on hardware, the statistical methods used to obtain error bars, and any cross-validation or tests for correlated noise or crosstalk that could invalidate the additive assumption.
  2. [Results] Results section (presumably §4 or equivalent): without explicit reporting of the raw hardware data points, circuit depths, measurement statistics, and data-exclusion criteria for the H₂ subsystems, the central claim that size consistency holds within chemical accuracy cannot be independently assessed from the presented text.
minor comments (2)
  1. [Abstract] Clarify the precise numerical threshold adopted for 'chemical accuracy' (e.g., 1.6 mHa or another value) and how it is applied per molecule versus total energy.
  2. [Methods] Provide a brief description or reference for the construction of the 'optimally shallow unitary circuits' to allow readers to reproduce the ansatz choice.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which highlight important aspects of clarity and reproducibility. We address each major comment below and will revise the manuscript accordingly to strengthen the presentation of our results.

read point-by-point responses
  1. Referee: [Abstract] Abstract: the headline limits of 118 and 71 H₂ subsystems are extrapolations obtained by fitting energy deviations on small numbers of molecules to a noise model and projecting forward; the manuscript must detail the functional form of the fit, the exact range of system sizes actually executed on hardware, the statistical methods used to obtain error bars, and any cross-validation or tests for correlated noise or crosstalk that could invalidate the additive assumption.

    Authors: We agree that the extrapolation procedure requires more explicit documentation. The functional form is a linear fit of the per-molecule energy deviation versus the number of H₂ subsystems, derived from an additive noise model. Hardware executions were performed for system sizes from 1 to the maximum feasible on the device (specifically up to 8 subsystems). Error bars were computed using bootstrap resampling of the shot-level measurement outcomes. We will add cross-validation by holding out subsets of the data for fit validation and include explicit checks for correlated noise via separate two-qubit crosstalk measurements on non-adjacent qubits. These details will be incorporated into the abstract and expanded in the Methods and Results sections. revision: yes

  2. Referee: [Results] Results section (presumably §4 or equivalent): without explicit reporting of the raw hardware data points, circuit depths, measurement statistics, and data-exclusion criteria for the H₂ subsystems, the central claim that size consistency holds within chemical accuracy cannot be independently assessed from the presented text.

    Authors: We acknowledge the need for greater transparency in the raw data and experimental parameters. In the revised manuscript we will add a dedicated table in the Results section listing the measured energies for each system size, the circuit depths (one-qubit and two-qubit unitary designs), the number of measurement shots per circuit, and the data-exclusion criteria (runs with variance exceeding a predefined threshold were discarded). This will enable independent verification of the size-consistency claims within chemical accuracy. revision: yes

Circularity Check

0 steps flagged

No significant circularity: results from direct hardware simulations

full rationale

The paper reports size-consistency findings from quantum hardware simulations of increasing numbers of non-interacting H2 molecules using optimally shallow unitary circuits. The abstract presents the 118 and 71 subsystem estimates as outcomes of these simulations, with the term 'estimated' referring to projection from observed energy deviations rather than any self-referential definition or tautological reduction. No equations, derivations, or load-bearing steps are shown that equate predictions to inputs by construction, smuggle ansatzes via self-citation, or rename known results. The central claim relies on external hardware benchmarks and noise considerations independent of the target figures, satisfying the default expectation of self-contained derivations without enumerated circular patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions from quantum computing and chemistry with no free parameters, invented entities, or ad-hoc axioms introduced in the abstract.

axioms (1)
  • domain assumption Quantum mechanics governs electron behavior in molecules and noise on quantum devices can couple independent subsystems
    Standard background assumption invoked to frame the size-consistency test.

pith-pipeline@v0.9.0 · 5452 in / 1230 out tokens · 31513 ms · 2026-05-16T20:35:36.971766+00:00 · methodology

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    Nielsen, M. A.; Chuang, I. L. Quantum Computation and Quantum Information; Cambridge University Press, 2000 mcitethebibliography main-arXiv.tex0000664000000000000000000010421615121526764012330 0ustar rootroot [journal=jpclck,manuscript=letter] achemso chemformula [T1] fontenc dsfont hyperref * [1] #1 Noah Garrett Michael Rose David A. Mazziotti Department...