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arxiv: 2604.17925 · v1 · submitted 2026-04-20 · 🪐 quant-ph · physics.chem-ph

State-Averaged Quantum Algorithms for Multiconfigurational Surface Chemistry: A Benchmark on Rh@TiO2(110)

Ernst Dennis L{\ae}gteskov Binau Larsson , Erik Kjellgren , Peter Reinholdt , Jacob Kongsted This is my paper

Pith reviewed 2026-05-10 05:34 UTC · model grok-4.3

classification 🪐 quant-ph physics.chem-ph
keywords state-averaged quantum algorithmsadaptive ansatzmulticonfigurational chemistrysurface catalysisCASSCF benchmarkNO adsorptionTiO2(110)quantum computing for chemistry
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The pith

The adaptive state-averaged quantum ansatz reaches near-CASSCF accuracy on a multiconfigurational NO adsorption model using far fewer operators than fixed-depth alternatives.

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

This paper benchmarks state-averaged factorized unitary coupled cluster and an adaptive ansatz against state-averaged CASSCF for an embedded cluster model of NO adsorbed on Rh-doped TiO2(110). The chosen system displays pronounced multiconfigurational character together with multiple state crossings, serving as a demanding test for quantum methods in realistic surface chemistry. SA-ADAPT attains reference-level accuracy with markedly fewer operators, while a revised selection rule that adds several operators each iteration accelerates convergence. The results indicate that adaptive constructions handle multistate problems more efficiently than standard fixed ansatze when applied to chemically motivated models beyond minimal cases.

Core claim

On the embedded Rh@TiO2(110) cluster with fixed orbitals from state-averaged CASSCF, SA-fUCCSD improves as circuit depth grows yet demands many parameters and remains sensitive to initialization, whereas SA-ADAPT reaches near-CASSCF accuracy with substantially fewer operators and a modified scheme that incorporates multiple operators per iteration speeds convergence markedly.

What carries the argument

State-averaged adaptive problem-tailored ansatz (SA-ADAPT) equipped with a multi-operator selection rule per iteration, used to solve the CASCI problem inside a fixed orbital basis taken from classical state-averaged CASSCF.

Load-bearing premise

The embedded cluster model of NO adsorption on Rh@TiO2(110) together with a fixed orbital basis from state-averaged CASSCF is sufficient to represent the multiconfigurational character and state crossings that would appear in the full periodic surface system.

What would settle it

Direct computation of state energies and crossing locations for the same active space on a periodic supercell model of the Rh@TiO2(110) surface, followed by comparison of energy errors and crossing geometries against the embedded-cluster results.

Figures

Figures reproduced from arXiv: 2604.17925 by Erik Kjellgren, Ernst Dennis L{\ae}gteskov Binau Larsson, Jacob Kongsted, Peter Reinholdt.

Figure 1
Figure 1. Figure 1: Visualization of NO adsorbed on Rh-doped TiO [PITH_FULL_IMAGE:figures/full_fig_p010_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: SA-CASSCF (10,8) adsorption profile for NO on Rh@TiO [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: CSF weights along the adsorption/desorption pathway for the three states shown [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Errors of SA-fUCCSD and SA-ADAPT relative to the SA-CASSCF reference [PITH_FULL_IMAGE:figures/full_fig_p014_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Convergence profiles at the equilibrium geometry with SA-ADAPT. [PITH_FULL_IMAGE:figures/full_fig_p016_5.png] view at source ↗
read the original abstract

Accurate modeling of surface catalytic processes often requires methods capable of describing strong correlation, charge transfer, and multiple closely lying electronic states. While density functional theory remains widely used, its limitations for localized electronic states motivate the use of wavefunction-based approaches and, more recently, quantum computing algorithms. However, the performance of quantum ans\"atze in chemically motivated, multistate settings remains largely unexplored. Here, we benchmark state-averaged factorized unitary coupled cluster with singles and doubles (SA-fUCCSD) and the adaptive, problem-tailored ansatz (SA-ADAPT) using an embedded cluster model of NO adsorption on Rh-doped TiO2(110). The system exhibits pronounced multiconfigurational character and multiple state crossings, providing a stringent test. State-averaged CASSCF serves as a reference, and the quantum ans\"atze are evaluated as solvers for the corresponding CASCI problem within a fixed orbital basis. We find that SA-fUCCSD improves with increasing circuit depth but requires many parameters and shows sensitivity to initialization. In contrast, SA-ADAPT achieves near-CASSCF accuracy with significantly fewer operators. A modified operator selection scheme, incorporating multiple operators per iteration, substantially accelerates convergence. Our results demonstrate the efficiency of adaptive ans\"atze for multistate problems and establish a controlled benchmark for quantum algorithms in chemically motivated systems beyond minimal models.

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

0 major / 3 minor

Summary. The paper benchmarks state-averaged factorized unitary coupled cluster (SA-fUCCSD) and the adaptive SA-ADAPT ansatz as solvers for the CASCI problem within a fixed orbital basis taken from state-averaged CASSCF. Using an embedded cluster model of NO adsorption on Rh@TiO2(110) chosen for its multiconfigurational character and state crossings, the work claims that SA-ADAPT reaches near-CASSCF accuracy with substantially fewer operators than SA-fUCCSD and that a modified operator selection scheme (multiple operators per iteration) accelerates convergence.

Significance. If the reported numerical comparisons hold, the work is significant as a controlled benchmark of adaptive quantum ansatze on a chemically motivated multistate system beyond minimal models. It directly demonstrates resource advantages of problem-tailored adaptive constructions over fixed ansatze such as fUCCSD for state-averaged problems, which is relevant for quantum algorithms targeting catalysis and strong-correlation surface chemistry.

minor comments (3)
  1. The abstract asserts 'near-CASSCF accuracy' and 'significantly fewer operators' without quoting concrete energy deviations, operator counts, or circuit depths; these quantitative metrics should appear in the abstract or be highlighted in the first paragraph of the results section to allow immediate evaluation of the central claim.
  2. The modified multi-operator selection scheme is described only qualitatively; a precise algorithmic statement (pseudocode or explicit rule for choosing and adding multiple operators per iteration) should be added to the methods section so that the reported acceleration can be reproduced.
  3. All energy tables or convergence plots should report both absolute and relative errors to the CASSCF reference together with the number of variational parameters or operators at each point, to make the 'fewer operators' comparison unambiguous.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive evaluation of the manuscript, accurate summary of our benchmark, and recommendation for minor revision. We appreciate the recognition that this work provides a controlled test of adaptive state-averaged quantum ansatze on a chemically relevant multistate surface system.

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper benchmarks SA-fUCCSD and SA-ADAPT quantum ansatze against an independent state-averaged CASSCF reference for a fixed CASCI problem in an explicitly defined embedded cluster model of NO on Rh@TiO2(110). All reported accuracies, operator counts, and convergence behaviors are direct numerical comparisons within that space; no equations reduce the results to fitted parameters, self-definitions, or self-citation chains. The embedded-cluster setup is presented as the test system itself rather than an approximation whose validity is derived from the quantum results. No load-bearing steps match the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review; no explicit free parameters, axioms, or invented entities are stated. The central claim rests on the domain assumption that CASSCF orbitals and energies constitute a faithful reference for the quantum ansatze.

axioms (1)
  • domain assumption State-averaged CASSCF supplies an accurate reference for the multiconfigurational CASCI problem in the embedded cluster.
    Used throughout as the benchmark against which both quantum ansatze are judged.

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