{"id":"7df09909-bda5-46c9-ae6b-872d291b4fc0","arxiv_id":"2608.09747","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Trajectory-averaged MR-ADC simulations reproduce the transient carbon K-edge X-ray absorption of acetylacetone and assign the early-time and 7-10 picosecond features to specific nuclear geometries and triplet core excitations.","lead":"This paper simulates time-resolved carbon K-edge X-ray absorption spectra of acetylacetone by combining surface-hopping molecular dynamics with multireference electronic structure calculations. The simulations reproduce the main experimental features and tie specific spectral peaks to transient proton-sharing geometries, bond alternation, and triplet-state orbital excitations.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Long-time T1 assignment rests on an equilibrium 300 K Wigner ensemble that is not dynamically connected to the S1 population from which intersystem crossing actually forms the triplet; this is the weakest load-bearing premise in the paper.","rationale":"The early-time and intermediate-time singlet part of the paper is internally well supported: trajectories directly generate the nuclear ensembles, spectral averaging is checked for convergence (30 vs 40 trajectories), and the proton-sharing analysis of the 279.5–281.5 eV feature is backed by both a correlation plot and natural transition orbitals. The long-time triplet assignment is the one place where the ensemble is not derived from the simulated dynamics, and the paper is transparent about this. The reader's weakest assumption correctly identifies this gap. My test targets exactly that premise: if a T1 ensemble born from S1 at the actual intersystem-crossing time gives the same 281.4 and 283.8 eV features as the equilibrium Wigner ensemble, the current assignment is adequate; if not, the central claim about triplet formation is not established. Because the reader's verdict was already CONDITIONAL, and my concern reinforces rather than overturns that judgment, the verdict is unchanged.","tokens_in":18636,"tokens_out":4484,"duration_ms":42816,"concrete_test":"Extend the surface-hopping simulations to include spin–orbit coupling (or, as a cheaper surrogate, initialize T1 trajectories from S1 geometries and velocities sampled at about 1.5 ps, the experimental intersystem-crossing time, instead of from the equilibrium T1 Wigner distribution), propagate these born T1 ensembles for at least 1–2 ps, and compute vibrationally averaged MR-ADC(2)-SX/cc-pwCVDZ carbon K-edge spectra. Compare the resulting peak positions and intensities of the 281.4 and 283.8 eV features with Figure 14. If the equilibrium-Wigner and intersystem-crossing-born ensembles agree within about 0.3 eV and 20% intensity, the current T1 ensemble is adequate; otherwise the long-time triplet assignment is not robust and should be re-evaluated.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's strongest claim includes the long-time (7–10 ps) assignment of the 281.4 and 283.8 eV features to C3/C4 1s excitations into triplet pi orbitals. That assignment depends on the triplet nuclear ensemble used in the calculation. Section 3 states that 'Spin–orbit coupling was not included in the singlet-state dynamics,' and Section 4.5 states that 'because the singlet-state nonadiabatic dynamics did not include spin–orbit coupling or intersystem crossing, we constructed a representative triplet-state ensemble by propagating independent trajectories on the T1 potential-energy surface.' This is not a dynamically generated T1 distribution: a 300 K harmonic Wigner ensemble around the CASPT2 T1 minimum, propagated for only 50 fs, assumes the triplet is vibrationally equilibrated at room temperature and located near the T1 minimum. In reality, T1 is formed by intersystem crossing from S1 at about 1.5 ps, so the initial T1 geometries and velocities inherit S1's proton-transfer, ring-opening, and twisting coordinates plus excess vibrational energy released in the S1-to-T1 transition, and the experimental signal is recorded 5.5–8.5 ps later. The paper itself demonstrates that core-excitation energies and intensities are highly geometry-dependent in the singlet manifolds; there is no reason to assume the T1 spectrum is insensitive to the same coordinates. If the true T1 distribution is shifted toward S1-like or hot geometries, the computed 281.4/283.8 eV peaks and their relative intensities could change, undermining the long-time assignment. A secondary issue is that the preferred production protocol (MR-ADC(2)) underestimates the 283.8 eV peak, and the recovery of this peak uses single-point MR-ADC(2)-SX without ensemble averaging. The ensemble disconnect is the more fundamental gap.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports time-resolved simulations of the carbon K-edge X-ray absorption spectrum of acetylacetone (AcAc), combining XMS-CASPT2 fewest-switches surface-hopping dynamics with core-valence-separated MR-ADC(2) calculations of C1s excitation spectra. The central claims are: (i) the early-time (20–200 fs) transient spectrum is governed by continuously evolving distributions of nuclear geometries on the S2 and S1 surfaces rather than by electronic-state populations alone; (ii) the weak 279.5–281.5 eV absorption (peak a) is a marker of transient, nearly symmetric proton-sharing (C2v-like) configurations; (iii) the 284–286 eV doublet arises from a broad geometry-dependent manifold of C1s excitations whose apparent splitting is largely caused by overlap with the ground-state bleach; and (iv) the long-time (7–10 ps) spectrum is dominated by triplet T1 features near 281.4 and 283.8 eV assigned to C3 and C4 1s excitations into low-lying triplet pi orbitals. The paper also introduces and benchmarks a semi-internal extended variant, MR-ADC(2)-SX, which improves the description of the triplet-state spectrum.","tokens_in":18956,"tokens_out":4112,"duration_ms":40041,"significance":"If the assignments hold, the paper provides a mechanistic connection between time-resolved carbon K-edge spectra and specific nuclear coordinates (proton transfer, bond alternation, ring opening, triplet formation), which is a valuable step beyond stationary-geometry interpretations. The work has clear strengths: it uses a multireference core-excitation method across nonequilibrium ensembles; it demonstrates the necessity of nuclear sampling for these spectra; it provides a state-resolved analysis; and it offers a falsifiable structural correlation (Figure 10) that is not purely an artifact of the fitted energy shifts. The early-time geometric assignments and the bleach-splitting argument are supported by internal consistency and by comparison with experiment. However, the long-time triplet assignment rests on an independently constructed T1 ensemble that is not dynamically connected to the S1 population, and the overall comparison with experiment relies on several fitted parameters (energy shifts, Lorentzian widths, intensity scaling). These issues weaken but do not invalidate the central mechanistic picture.","major_comments":[{"comment":"The comparison with experiment uses multiple fitted parameters: global energy shifts (−3.5 eV for single-point, −3.25 eV for vibrationally averaged, and a different −2.0 eV for the MR-ADC(2)-SX spectrum in Figure 15), Lorentzian broadening widths (0.4 and 0.2 eV), and a uniform intensity scaling factor. These are fitted to the same experimental TR-XAS spectra being interpreted, which adds a circularity burden to statements such as 'good agreement' and 'markedly improved description.' The central geometric correlation (Figure 10) and the bleach-splitting argument are less affected because they rely on relative intensities and energy trends, but the absolute peak-energy assignments and the quantitative claims of agreement should be couched more carefully. In particular, the use of a different energy shift for the MR-ADC(2)-SX triplet spectrum means that part of the observed improvement in Figure 15 is absorbed by the extra shift parameter; the text should state this explicitly and discuss the sensitivity of the assignment to the shift choice.","section":"Section 3, Figures 14 and 15"}],"minor_comments":[{"comment":"Typo: 'non-equillibrium' should be 'non-equilibrium.'","section":"Figure 11 caption"}],"recommendation":"major_revision","confidential_remarks":"The core early-time assignment is solid and the multireference trajectory-based protocol is a genuine methodological contribution. The main risk is the long-time T1 assignment: the separate Wigner-ensemble construction is an acknowledged approximation, but it is load-bearing for the paper's most striking claim (triplet-state feature assignment). I would like the editor to ensure the authors either provide additional calculations probing the sensitivity of the triplet spectrum to the initial ensemble or explicitly downgrade the claim. The fitted energy shifts are a secondary but real concern; however, they are common practice in core-level spectroscopy and do not by themselves invalidate the geometric conclusions."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things to know. First, this is the first study to combine XMS-CASPT2 surface-hopping dynamics with MR-ADC core-excited spectra for a TR-XAS problem, and it uses that combination to argue that the transient carbon K-edge spectrum of acetylacetone is shaped by continuously evolving geometry distributions, not just electronic-state populations. That argument is mostly convincing. Second, the long-time triplet assignment is weaker than the singlet part, and the reason is exactly the stress-test note: the T1 ensemble is an equilibrium Wigner distribution around the T1 minimum propagated for 50 fs, disconnected from the S1 population because spin-orbit coupling is absent. So the 281.4/283.8 eV assignment is a reasonable candidate, not a settled one.\n\nWhat the paper does well: the ground-state benchmark is thorough (eight active spaces, three MR-ADC variants, three basis sets) and the choice of MR-ADC(2)/cc-pwCVDZ is justified. The proton-sharing marker is a real result: the correlation between O-H/H-O bond asymmetry and intensity in the 279.5-281.5 eV window across 560 spectra is a clean, internally consistent analysis, and the NTO explanation (localization on C3) is sensible. The bleach-splitting interpretation of the 284-286 eV doublet is also a good, parameter-free check: removing the ground-state bleach merges the two apparent maxima, which directly tests the assignment. The introduction of MR-ADC(2)-SX is a useful intermediate, though its main success here is a single-point triplet spectrum, so it is not yet heavily validated.\n\nSoft spots. The T1 ensemble problem is load-bearing. ISC in AcAc is linked to out-of-plane twisting, and the paper itself shows core-excitation energies are geometry-sensitive in the singlet manifolds. A 300 K harmonic Wigner ensemble around the T1 minimum assumes the triplet is thermalized and near the minimum; a hot or S1-like T1 distribution could shift the computed peaks. This is a genuine gap, and the paper states it plainly, but it is not a minor caveat. The fitted energy shifts (-3.5, -3.25, -2.0 eV) and Lorentzian widths mean the comparison with experiment is alignment-dependent; standard practice in core-level spectroscopy, but it does weaken any claim that absolute energies are reproduced. The intensity of peak 2 requires SX rather than the production method; the lack of ensemble averaging for that single-point spectrum is another reason to treat the triplet part cautiously. None of this sinks the singlet assignments, which are supported by trajectory-resolved analysis rather than by fitting.\n\nWho should read it: anyone simulating TR-XAS or using MR-ADC, and experimentalists working on acetylacetone. It deserves serious peer review. I would send it out, with a request that the authors either run T1 trajectories from S1/T1 crossing geometries or explicitly test sensitivity to the T1 initial conditions. The paper is honest about its own limitation, but the limitation should be addressed or bounded before the long-time assignment is taken as definitive.","headline":"Solid singlet-side story with a genuinely new method combination and a clean marker for proton-sharing geometries, but the long-time triplet assignment rests on a thermalized T1 ensemble that the dynamics do not actually produce.","tokens_in":19547,"tokens_out":4529,"would_cite":true,"duration_ms":29570,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The transient carbon K-edge spectrum of acetylacetone is governed by evolving proton-sharing geometries on the $S_2$ and $S_1$ surfaces, with the long-time triplet features assigned to central-carbon 1s excitations into low-lying triplet…","keywords":["transient X-ray absorption spectroscopy","acetylacetone","multireference algebraic diagrammatic construction","core-valence separation","excited-state intramolecular hydrogen transfer","surface-hopping dynamics","carbon K-edge spectrum","triplet state"],"falsifier":"A computational experiment that propagates the $S_2$/$S_1$ dynamics while biasing the enolic proton to remain localized on one oxygen should eliminate the 279.5-281.5 eV band; if the band persists in such an ensemble, the paper's assignment to proton-sharing geometries would be falsified.","tokens_in":18401,"feed_emoji":"⚛️","tokens_out":11701,"duration_ms":94729,"temperature":0.7,"pith_summary":"The paper reports trajectory-averaged simulations of the transient carbon K-edge X-ray absorption spectrum of acetylacetone, combining multireference algebraic diagrammatic construction (MR-ADC) for core-excited states with surface-hopping dynamics that generate nonequilibrium molecular geometries. It argues that the 20-200 fs spectrum is shaped by continuously evolving distributions of structures on the $S_2$ and $S_1$ surfaces, not by a handful of stationary geometries. The weak absorption at 279.5-281.5 eV is traced to transient, nearly symmetric proton-sharing configurations, while the 284-286 eV envelope reflects C1s excitations modulated by proton transfer, bond alternation, and ring opening. At 7-10 ps, the principal peaks are assigned to central-carbon 1s excitations into low-lying triplet π orbitals, with the second peak's intensity recovered only when the extended semi-internal MR-ADC(2)-SX approximation is used.","feed_headline":"Proton sharing drives acetylacetone's transient X-ray signal","feed_subtitle":"Ensemble simulations decode acetylacetone's carbon K-edge spectrum and assign the late triplet peaks.","key_machinery":"The load-bearing machinery is the ensemble-averaged MR-ADC spectrum: multireference algebraic diagrammatic construction (MR-ADC) with the core-valence separation approximation, applied to geometries sampled from fewest-switches surface-hopping trajectories driven by multistate complete active space second-order perturbation theory. The specific protocol MR-ADC(2)/CASSCF(10e,8o)/cc-pwCVDZ was selected by benchmarking the ground-state carbon K-edge spectrum. The central observable is the correlation between the O-H/H-O bond-length deviation (how symmetric the proton-sharing configuration is) and the computed intensity at 279.5-281.5 eV, together with natural transition orbital analysis (which represents each excitation by its dominant occupied-virtual orbital pair) showing that the intensity tracks the spatial overlap of the C3 1s core orbital with the accepting π-type orbital.","core_discovery":"The central discovery is that the transient X-ray absorption of acetylacetone is governed by the evolving nuclear geometry distribution on the singlet $S_2$ and $S_1$ states rather than by electronic-state populations or single-point spectra alone. At 20-100 fs, absorption between 279.5 and 281.5 eV is enhanced specifically for nearly symmetric, $C_{2v}$-like proton-sharing structures formed during excited-state intramolecular hydrogen transfer; the enhancement arises because the accepting π-type orbital localizes on the central carbon C3, increasing its overlap with the C3 1s core orbital. The 284-286 eV band is a single broad, geometry-dependent manifold of C1s transitions that appears doublet-like only because of overlap with the ground-state bleach near 285 eV. In the long-lived triplet spectrum, the features near 281.4 and 283.8 eV are assigned to C3 and C4 1s excitations into low-lying triplet π orbitals, and the weaker intensity of the second feature under MR-ADC(2) is corrected by the MR-ADC(2)-SX approximation.","pith_inferences":["Editorial inference: the proton-sharing marker should generalize to other intramolecularly hydrogen-bonded enols; substituting the bridging proton with deuterium would be a direct test, since the zero-point spread along the O-H-O coordinate should change the 279.5-281.5 eV intensity.","Editorial inference: because the singlet trajectories excluded spin-orbit coupling, the simulated $T_1$ ensemble may not capture the vibrational distribution at the moment of intersystem crossing; including spin-orbit couplings in the dynamics would test whether the 7-10 ps spectrum survives a realistic crossing distribution.","Editorial inference: the bleach-overlap explanation predicts that the apparent 284-286 eV doublet should shift as the ground-state population recovers, so time-resolved analysis after separating the bleach contribution could expose the underlying state-specific envelope.","Editorial inference: the same ensemble-averaged MR-ADC pipeline could be extended to other ultrafast carbon K-edge experiments where multiconfigurational excited states and nonadiabatic motion coexist, provided the active space captures the relevant valence and core orbitals."],"forward_implications":["The 279.5-281.5 eV absorption can serve as a real-time marker of the short-lived proton-sharing geometry reached during excited-state intramolecular hydrogen transfer.","The apparent doublet at 284-286 eV is not two distinct excited-state bands; it is one broad excited-state envelope split by the ground-state bleach, so transient difference spectra should be interpreted with the bleach included.","Single-point calculations at stationary geometries are insufficient for assigning TR-XAS of acetylacetone; vibrationally averaged ensembles change peak positions, widths, and intensities.","The long-time 7-10 ps spectrum is dominated by central-carbon (C3 and C4) 1s excitations into low-lying triplet π orbitals of $T_1$.","For open-shell triplet core spectra, MR-ADC(2) underestimates the 283.8 eV intensity, and the semi-internal extended variant MR-ADC(2)-SX is needed to reproduce the experimental peak ratio."],"supporting_citations":[{"why":"supplies the experimental TR-XAS spectra of AcAc at 20-100 fs, 120-200 fs, and 7-10 ps that the simulations reproduce and assign.","marker":"[8]"},{"why":"supplies the CASPT2 ground-state and excited-state geometries, the experimental population dynamics used for comparison, and the timescales that anchor the relaxation mechanism.","marker":"[9]"},{"why":"supplies independent population-dynamics and structural-dynamics data that support the sequential S2 to S1 to S0 relaxation pathway.","marker":"[12]"},{"why":"introduces the MR-ADC theory on which all core-excitation spectra are based.","marker":"[33]"},{"why":"introduces the core-valence separation approximation used to target carbon 1s excitations.","marker":"[38]"},{"why":"provides the fewest-switches surface-hopping algorithm used to generate the nuclear ensembles.","marker":"[60]"},{"why":"provides the extended multistate CASPT2 electronic-structure method that drives the surface-hopping trajectories and the reoptimized geometries.","marker":"[67]"}],"fun_headline_variants":["Proton sharing fingerprints acetylacetone's transient states","X-ray spectra decode acetylacetone's proton dynamics","Proton transfer imprinted in transient X-ray spectra","Transient X-rays capture acetylacetone proton-sharing","Acetylacetone's X-ray signal reveals proton-sharing regime"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The long-time triplet assignment rests on assuming that thermally sampled geometries around the relaxed $T_1$ minimum, propagated for 50 fs, faithfully represent the actual triplet molecules formed by intersystem crossing, even though the singlet trajectories carried no spin-orbit coupling and therefore never generated the triplet population dynamically.","fun_headline_variants_meta":{"raw":{"variants":["Proton sharing fingerprints acetylacetone's transient states","X-ray spectra decode acetylacetone's proton dynamics","Proton transfer imprinted in transient X-ray spectra","Transient X-rays capture acetylacetone proton-sharing","Acetylacetone's X-ray signal reveals proton-sharing regime"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00024,"raw_usage":{"total_tokens":1565,"prompt_tokens":1040,"completion_tokens":525,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":656,"completion_tokens_details":{"reasoning_tokens":447}},"tokens_in":656,"tokens_out":525,"duration_ms":4767,"temperature":1.0,"reasoning_tokens":447,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T11:30:47.412890+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A computational experiment that propagates the $S_2$/$S_1$ dynamics while biasing the enolic proton to remain localized on one oxygen should eliminate the 279.5-281.5 eV band; if the band persists in such an ensemble, the paper's assignment to proton-sharing geometries would be falsified.","supporting_citations":[],"review_version":1}