{"id":"78844068-95f7-4273-8ae7-cf4764fcedf3","arxiv_id":"2507.22670","paper_version":3,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"Variational quantum algorithms reproduce an avoided crossing in methanimine, but the water conical intersection they claim comes from a classical calculation, not from running a quantum algorithm at that geometry.","lead":"The paper simulates variational quantum algorithms on small active spaces to locate conical intersections, points where two molecular energy surfaces meet, in water and methanimine. It finds the quantum methods reproduce the methanimine avoided crossing well, while the water intersection is located only by a classical state-average calculation.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Water conical-intersection claim rests on a classical SA-CASSCF calculation at a geometry where no variational quantum algorithm was run; the abstract's 'both molecules' overstates the evidence.","rationale":"The reader's weakest_assumption identifies exactly the load-bearing issue: the water conical intersection is demonstrated only by a classical state-average CASSCF calculation, with no quantum variational algorithm run at that geometry, and the authors themselves note the result's sensitivity to basis set and active space. I agree with that assessment. The paper does contain a genuine positive result for methanimine—VQD and VQE-AC energies track the exact avoided-crossing curves in the (4,3) active space—so the work is not empty. But the abstract's blanket claim about 'both molecules' is not supported by the reported experiments. A CONDITIONAL verdict is appropriate: the claim should be narrowed to methanimine, or a water VQA simulation should be added, and the classical nature of the water result should be stated in the abstract and conclusion. The proposed decisive test is to perform the missing quantum simulation at the water crossing geometry; if that is computationally infeasible, a basis/active-space sensitivity check would at least test the stability of the classical CI on which the claim rests.","tokens_in":16491,"tokens_out":4334,"duration_ms":56729,"concrete_test":"Run VQD or VQE-AC at the water Jacobi geometry with G=0.25 Å, r=2.5832 Å, γ=0.00021°, using the reported (10,12) active space and 6-31G basis, and compare the ground- and first-excited-state gap against the SA-CASSCF result. If the quantum algorithm cannot reproduce the vanishing gap, or if the gap remains finite, the abstract's 'both molecules' claim fails. As a necessary cheaper check, recompute the SA-CASSCF crossing with 6-31G* or aug-cc-pVDZ; if the G≈0.25 Å degeneracy disappears, the classical foundation for the water claim is an artifact regardless of any quantum simulation.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that variational quantum algorithms can describe conical intersections in both CH2NH and H2O. For CH2NH, VQD and VQE-AC runs at a (4,3) active space do reproduce the exact-diagonalization avoided crossing near α=94°. For H2O, however, the only conical-intersection evidence is the classical state-average CASSCF calculation in Experiment 4 (Section IV.4 and Fig. 12) at a heavily deformed Jacobi geometry with r=2.5832 Å, γ=0.00021°, and a crossing near G=0.25 Å. The authors explicitly state that 'changes to the basis set or the active space could make the CI disappear in the SA results' and that 'simulations of quantum algorithms applied on these larger systems ... could be addressed in future works.' No VQE, VQD, or VQE-AC result is reported at that geometry. The load-bearing assumption is therefore that a classical, basis/active-space-sensitive energy degeneracy would transfer to a variational quantum simulation. That assumption is untested and admitted to be fragile. The abstract and conclusion credit the quantum variational algorithms with describing conical intersections 'in both molecules,' even though the water half of that statement is supported only by a classical calculation flagged by the authors as unstable. A related limitation is that even the CH2NH quantum results demonstrate an avoided crossing rather than a true degenerate conical intersection, so the phrase 'conical intersections' is used loosely throughout.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports simulations of variational quantum algorithms (VQE, VQD, VQE-AC) and classical state-averaged CASSCF on the water and methanimine molecules, aiming to locate conical intersections. For CH2NH, VQD and VQE-AC calculations at a (4,3) active space reproduce an avoided crossing obtained by exact diagonalization near a bending angle of α≈94°, and a (12,9) active-space VQD run reproduces the qualitative trend with larger systematic errors. For H2O, VQE/VQD/VQE-AC are validated on symmetric O–H stretch deformations, and a conical intersection is claimed based solely on a classical SA-CASSCF calculation at a heavily deformed Jacobi geometry (G≈0.25 Å). The abstract and conclusion assert that the work 'confirms the quantum variational algorithms' capability of describing conical intersections in both molecules.'","tokens_in":16803,"tokens_out":6011,"duration_ms":71234,"significance":"If the claims were fully supported, this would be a useful benchmark of excited-state variational quantum algorithms against exact diagonalization, particularly the CH2NH avoided-crossing result at α=94°±2° and the comparison of VQD with VQE-AC. The explicit external benchmark, the clear experimental setup, and the demonstration of ansatz/optimizer sensitivity are strengths. However, the headline claim about 'both molecules' is not supported by the reported quantum simulations: the water conical intersection is identified only by classical SA-CASSCF, and the CH2NH results demonstrate an avoided crossing along a one-dimensional scan rather than a true conical intersection. The paper also lacks a data/code availability statement, limiting reproducibility. With a corrected scope, the study would be a modest but legitimate contribution to the VQA-for-excited-states literature.","major_comments":[{"comment":"The abstract claims 'This work confirms the quantum variational algorithms' capability of describing conical intersections in both molecules,' but the only evidence for a conical intersection in water is a classical SA-CASSCF calculation at a (10,12) active space with the 6-31G basis set (Fig. 12). No VQE, VQD, or VQE-AC simulation is reported at the Jacobi geometry; the text explicitly states 'Simulations of quantum algorithms applied on these larger systems ... could be addressed in future works' and 'Changes to the basis set or the active space could make the CI disappear in the SA results.' The abstract and conclusion therefore overstate the support for the water half of the central claim. Please either report a variational quantum simulation at this geometry (or a reduced variant) or revise the abstract and conclusion to state that the water CI is identified by a classical state-averaged calculation and proposed as a target for future quantum-simulation studies.","section":"Abstract; Section IV.4; Fig. 12; Section VI"},{"comment":"For CH2NH, the variational quantum algorithms produce energy curves along the single bending coordinate α with a minimum gap near α≈94°; they do not locate a true degeneracy in a two-dimensional branching space. The paper itself defines a conical intersection as a degeneracy of adiabatic potential energy surfaces (Section II.2) and notes that an avoided crossing is 'often interpreted as evidence for an underlying conical intersection' but is not itself a conical intersection. Consequently, the abstract's wording 'capability of describing conical intersections' is too strong for the CH2NH results. Please either perform a two-dimensional scan (e.g., vary α and a second internal coordinate) to demonstrate an actual crossing point, or consistently use 'avoided crossing indicative of a conical intersection' for the quantum-algorithm results in the abstract, main text, and conclusion.","section":"Section V, Figs. 8–11; Section II.2; Section VI"},{"comment":"The (12,9) VQD results in Fig. 10 exhibit three distinct local minima, systematic upward energy shifts relative to exact diagonalization, and a gap at α≈90° that the authors describe as 'notably closer' but not closed. The conclusion nonetheless states that 'larger active spaces significantly improved the precision and expressiveness of quantum simulations in the vicinity of conical intersections.' This is not supported by the reported data: the (12,9) energies are less accurate in absolute terms than the (4,3) results, and no quantitative metric (e.g., minimum energy gap, state overlap, or gradient norm) is provided to demonstrate improvement near the crossing region. Please either quantify the claimed improvement or soften the conclusion to reflect the ansatz limitations and the absence of demonstrated improvement at the larger active space.","section":"Fig. 10; Section V; Section VI"}],"minor_comments":[{"comment":"The term 'Jacobbi' should be 'Jacobi' in the text and figure captions.","section":"Section IV.4"},{"comment":"The summation in Eq. (9) runs i=1 to k−1, but the text then refers to 'previously found k states (from the ground state, i=0, up to the (k−1)th excited state)'; please clarify the indexing convention for the previously computed states in the VQD penalty term.","section":"Eq. (9) and following text"},{"comment":"The target value of −74.96742 Hartree is not tied to a specific basis set and active space in the text; please state the computational setup corresponding to this exact energy.","section":"Fig. 4 and Section V (H2O VQE)"},{"comment":"The comparison between VQD (Efficient SU2) and VQE-AC (UCCSD) for CH2NH is not controlled with respect to the ansatz; please note this as a limitation or perform a same-ansatz comparison to isolate the effect of the orthogonality constraint.","section":"Section V, CH2NH VQD vs VQE-AC"},{"comment":"No data or code availability statement is provided, and software versions, optimizer settings, and random seeds are not specified; adding these would substantially improve reproducibility.","section":"General"}],"recommendation":"major_revision","confidential_remarks":"The paper contains a clean, externally benchmarked avoided-crossing result for CH2NH with VQD and VQE-AC, which is a legitimate contribution. The main problem is the abstract and conclusion's 'both molecules' claim, which is unsupported because the water result is purely classical and basis/active-space sensitive. The authors can likely fix this by carefully rescoping the claims and, if feasible, adding a 2D scan or a quantum simulation at a reduced water geometry. No concerns about citation ethics or novelty disclosure; the self-citation to their prior VQE study is appropriate."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: the methanimine part is a real, clean demonstration that excited-state VQAs can trace an avoided crossing in a small active space. The abstract's \"both molecules\" claim, though, is not supported: the water conical intersection is a classical SA-CASSCF finding at a deformed geometry where no quantum algorithm was ever run, and the authors themselves say the crossing may vanish with a change of basis or active space.\n\nWhat's genuinely useful here: the (4,3) CH2NH benchmark against exact diagonalization is clear, and the VQD versus VQE-AC comparison is a fair test of two variational excited-state methods. The (12,9) VQD results are honest enough to show the optimization artifacts and upward energy shifts, which is more than many benchmark papers do. The VQE-AC results are cleaner than VQD's, which is a legitimate point in favor of that method.\n\nThe soft spots are real but mostly fixable. First, the abstract and conclusion credit quantum variational algorithms with describing CIs \"in both molecules,\" but the water evidence is a classical state-average calculation; the paper itself flags the sensitivity. Second, prior quantum simulations of conical intersections exist and are not cited, so the novelty framing overreaches. Third, no code or data is provided, and the choice of the very small angle γ=0.00021° for the water Jacobi geometry looks hand-picked to force a near-degeneracy. Minor: the paper calls the CH2NH feature an avoided crossing and then a conical intersection interchangeably; the results actually demonstrate an avoided crossing, which is an indicator, not a proof of a true CI.\n\nThe circularity burden is low—the hand-chosen hyperparameters (β, constraint threshold) are not fitted to the target energies, and the benchmark against exact diagonalization is external. So the methanimine result is probably correct.\n\nWho should read it: people benchmarking VQD/VQE-AC on small molecules, and anyone wanting a cautious example of using VQAs to locate avoided crossings. It does not establish practical quantum simulation of conical intersections, but it is a reasonable proof-of-principle.\n\nFor peer review: I'd send it out. The core benchmark is reproducible and the overclaim is a rewording away from being addressed; a referee could ask for the water quantum simulation or a downgraded claim, and either way the paper deserves the engagement.","headline":"The methanimine benchmark is clean and useful; the 'both molecules' claim overreaches because the water conical intersection rests on a classical SA-CASSCF calculation with no quantum simulation behind it.","tokens_in":17368,"tokens_out":1997,"would_cite":true,"duration_ms":21015,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The paper claims that variational quantum algorithms—VQE, VQD, and VQE-AC—can describe conical intersections in water and methanimine when the active space and molecular geometry are chosen appropriately, locating a CH2NH avoided crossing…","keywords":["conical intersections","avoided crossings","variational quantum eigensolver","excited states","quantum chemistry","state-average CASSCF","methanimine","water molecule"],"falsifier":"Recompute the water ground and first excited states in the same Jacobi coordinates with a slightly different basis set (for example cc-pVDZ instead of 6-31G) or a different active space and check whether the gap at $G = 0.25$ Å stays closed, and run VQD or VQE-AC at that geometry to see if the degeneracy reproduces. For methanimine, compute the two-state gap at the (12,9) level with a more expressive ansatz or a multireference method and see whether the avoided crossing at $\\alpha \\approx 94^\\circ$ collapses to a true degeneracy or moves.","tokens_in":16297,"feed_emoji":"⚛️","tokens_out":8520,"duration_ms":84670,"temperature":0.7,"pith_summary":"This paper tries to establish that near-term variational quantum algorithms can locate conical intersections and avoided crossings in small molecules, as long as the active space and nuclear geometry are chosen to make the crossing visible. VQE, VQD, and VQE-AC are run on water and methanimine, benchmarked against exact diagonalization, with classical state-average CASSCF used to scan for degenerate regions. The concrete findings are a methanimine avoided crossing at $\\alpha = 94^\\circ \\pm 2^\\circ$ at the (4,3) active space, and a water degeneracy at $G = 0.25$ Å in a deformed Jacobi geometry. This matters because nonadiabatic regions control photochemistry and photophysics, and if these variational methods can map them on near-term hardware, the experimental bottleneck for studying such processes is lowered.","feed_headline":"Variational quantum algorithms pinpoint crossings in two molecules","feed_subtitle":"Choosing the right active space and geometry puts CH2NH's crossing near 94° and water's at G=0.25 Å.","key_machinery":"The load-bearing machinery is a set of variational cost functions plus one classical scanning tool. VQE minimizes the energy expectation value $\\langle\\Psi(\\theta)|H|\\Psi(\\theta)\\rangle$; VQD adds orthogonality penalties $\\beta_i\\,|\\langle\\Psi(\\theta_k)|\\Psi(\\lambda_i)\\rangle|^2$ to reach excited states; VQE-AC keeps the bare energy expectation value and enforces orthogonality externally as $|\\langle\\Psi(\\theta)|\\Psi_0\\rangle|^2 \\le 10^{-4}$; and state-average CASSCF averages a few electronic states together so that near-degeneracies become visible. The simulations use parity mapping to turn the fermionic Hamiltonian into qubit operators, the Efficient SU2 and UCCSD ansätze, and the SPSA, SLSQP, and COBYLA optimizers. The VQE-AC constraint, rather than a tuned penalty, is what the paper credits with stable behavior in the sensitive crossing region.","core_discovery":"On the paper's own terms, the central discovery is that variational quantum eigensolvers—VQE for ground states, VQD and VQE-AC for excited states—reproduce the exact-diagonalization energy surfaces closely enough to expose a conical-intersection signature in methanimine and, through a classical state-average CASSCF scan, in water. For CH2NH in the geometry with a $90^\\circ$ dihedral angle, VQD and VQE-AC both place the avoided crossing at $\\alpha = 94^\\circ \\pm 2^\\circ$; enlarging the active space from (4,3) to (12,9) narrows the ground–excited gap near $\\alpha \\approx 90^\\circ$ but shifts all energies upward because the ansatz struggles to express the larger space. For water, no crossing appears under symmetric O–H scaling; only the asymmetric Jacobi deformation with $\\gamma = 0.00021^\\circ$ produces an intersection, around $G = 0.25$ Å. The paper therefore concludes that the capability of the quantum algorithms to describe conical intersections is real but conditional on active-space and geometry choices.","pith_inferences":["A direct test the paper leaves implicit: run VQD or VQE-AC at the water Jacobi geometry near $G = 0.25$ Å to see whether the quantum algorithms actually close the gap the way the classical SA-CASSCF scan does.","A true conical intersection requires degeneracy in a two-dimensional branching space; confirming the CH2NH feature would mean scanning a second coordinate, such as the dihedral angle, and showing the crossing survives.","Because the paper notes the water CI disappears under basis-set or active-space changes, practical NISQ implementations there would likely need error mitigation and a carefully chosen active space to keep the intersection from washing out.","The (12,9) VQD artifacts and energy shifts suggest active-space size alone is not the bottleneck; ansatz expressiveness and optimizer behavior matter, so adaptive ansätze are a natural next step."],"forward_implications":["At the (4,3) active space, both VQD and VQE-AC put the CH2NH avoided crossing at $\\alpha \\approx 94^\\circ$, and the two algorithms agree with each other and with exact diagonalization near the crossing.","VQE-AC can replace VQD in crossing regions without tuning a penalty weight $\\beta$, because it enforces orthogonality as a hard constraint.","Symmetric water geometries never show a crossing, while the asymmetric Jacobi deformation does; geometry choice, not just method, determines whether a conical intersection is visible.","Larger active spaces improve the description of the crossing but raise computational cost and expose the limits of the chosen ansatz, as seen in the upward-shifted (12,9) CH2NH curves.","State-average CASSCF at the (4,3) active space reproduces the CH2NH near-degeneracy (around $\\alpha \\approx 100^\\circ$) at lower cost than exact diagonalization, making it a practical scan tool for choosing where to run quantum algorithms."],"supporting_citations":[{"why":"Supplies the VQE-AC algorithm with automatically-adjusted constraints that the paper uses as the simpler alternative to VQD.","marker":"[33]"},{"why":"Supplies the VQD algorithm whose penalty-based cost function produces the CH2NH avoided-crossing results.","marker":"[28]"},{"why":"Introduces the state-averaged approach the paper uses for the classical SA-CASSCF scans of both molecules.","marker":"[30]"},{"why":"Provides the deformed Jacobi geometry (r fixed, gamma=0.00021 degrees) where the water conical intersection is found.","marker":"[62]"},{"why":"Underpins the interpretation that an avoided crossing with an insufficient active space indicates a hidden conical intersection.","marker":"[48]"},{"why":"Defines the VQE framework that VQD and VQE-AC extend for ground- and excited-state energies.","marker":"[17]"},{"why":"Provides the quantum-chemistry implementation used for the state-average water calculation at G=0.25 angstrom.","marker":"[42]"}],"fun_headline_variants":["Quantum eigensolvers reveal molecular crossing points","VQE-based algorithms pinpoint conical intersections","Quantum simulation locates crossings in water and CH2NH","Hybrid quantum algorithms find molecular crossings"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The claim that quantum variational algorithms can describe the water conical intersection rests on a classical state-average CASSCF calculation at one heavily deformed Jacobi geometry, and the paper never runs a quantum algorithm at that geometry; the paper admits the intersection can disappear if the basis set or active space changes.","fun_headline_variants_meta":{"raw":{"variants":["Quantum eigensolvers reveal molecular crossing points","VQE-based algorithms pinpoint conical intersections","Quantum simulation locates crossings in water and CH2NH","Hybrid quantum algorithms find molecular crossings"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000588,"raw_usage":{"total_tokens":2780,"prompt_tokens":983,"completion_tokens":1797,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":599,"completion_tokens_details":{"reasoning_tokens":1740}},"tokens_in":599,"tokens_out":1797,"duration_ms":16294,"temperature":1.0,"reasoning_tokens":1740,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T11:24:11.776472+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Recompute the water ground and first excited states in the same Jacobi coordinates with a slightly different basis set (for example cc-pVDZ instead of 6-31G) or a different active space and check whether the gap at $G = 0.25$ Å stays closed, and run VQD or VQE-AC at that geometry to see if the degeneracy reproduces. For methanimine, compute the two-state gap at the (12,9) level with a more expressive ansatz or a multireference method and see whether the avoided crossing at $\\alpha \\approx 94^\\circ$ collapses to a true degeneracy or moves.","supporting_citations":[{"cited_title":"Excited state calculations using variational quantum eigensolver with spin-restricted ans¨ atze and automatically-adjusted constraints","cited_arxiv_id":null,"evidence_quote":"Supplies the VQE-AC algorithm with automatically-adjusted constraints that the paper uses as the simpler alternative to VQD."},{"cited_title":"Stanton and Rodney J","cited_arxiv_id":null,"evidence_quote":"Introduces the state-averaged approach the paper uses for the classical SA-CASSCF scans of both molecules."},{"cited_title":"Bernal Neira, and Mo- hamed Taha Rouabah","cited_arxiv_id":null,"evidence_quote":"Provides the deformed Jacobi geometry (r fixed, gamma=0.00021 degrees) where the water conical intersection is found."},{"cited_title":"Progress and challenges in the calculation of electronic excited states","cited_arxiv_id":null,"evidence_quote":"Underpins the interpretation that an avoided crossing with an insufficient active space indicates a hidden conical intersection."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the quantum-chemistry implementation used for the state-average water calculation at G=0.25 angstrom."}],"review_version":1}