{"id":"4b4bfc26-206a-4b67-9110-54effd2112ee","arxiv_id":"2508.20608","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"The orientation of the bridging hydroxyl groups in MIL-120(Al) is a hidden structural degree of freedom that controls CO2 adsorption geometry and energetics, with interconversion barriers low enough to be active at room temperature.","lead":"This paper uses density functional theory and a machine-learned potential to map six possible orientations of the bridging hydroxyl groups in the aluminum MOF MIL-120(Al), and shows that these orientations change how CO2 molecules sit in the pores. The result matters because most prior simulations assumed one fixed hydroxyl arrangement, which can misrepresent CO2 capture energetics in ultra-small pore MOFs.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Energy ranking of the six μ2-OH configurations rests entirely on PBE-D3 and a hand-selected set, with no test that the ordering survives a second functional.","rationale":"The reader's identified weakest assumption—the reliability of the PBE-D3 energy ranking over a hand-selected set of six μ2-OH patterns—is exactly the load-bearing concern I find. This is a computational study with no experimental H-position data (X-ray cannot see H), so the entire narrative of a hidden structural degree of freedom, the ground-state ordering, and the CO2-placement control depends on the DFT functional's description of H-bonded configurations and on the completeness of the six-configuration enumeration. The paper makes an internal consistency argument (DFT vs MLP, phonon stability, Qst match), but internal consistency of a single functional does not establish that the barrier heights and ordering are physical. The Qst match against experiment is the strongest independent evidence, but it is only meaningful if the A/B/C configurations are truly low-energy and if the configurational space has been sampled representatively; with an incomplete hand-built set, the Qst agreement could in principle be fortuitous. The concern does not overturn the paper because the direction of the effect—μ2-OH orientation modulating pore aperture and CO2 binding—is plausible and directly demonstrated within the chosen model chemistry, and the DFT-level evidence is competently cross-checked. However, the quantitative central claims (0.59 eV spread, 0.07–0.19 eV barriers, A ground state) are not yet anchored against functional-choice sensitivity. I therefore keep the reader's CONDITIONAL verdict: the work is acceptable as a computational study conditional on the stated caveats, and adding a second functional and an explicit completeness check would materially strengthen the conclusion. My agreement_with_reader is 'agree' because the reader's weakest_assumption directly identifies the PBE-D3/hand-selection issue as the key risk.","tokens_in":9202,"tokens_out":1861,"duration_ms":16115,"concrete_test":"Recompute the relative energies of the six empty MIL-120(Al) configurations (and the A↔F CI-NEB barrier, or at least the A↔B and A↔F barriers) with a second DFT functional, e.g., r2SCAN-D3 or PBE0-D3, using the same plane-wave settings and the same fully relaxed geometries. If the A-configuration remains lowest by a margin larger than ~0.1 eV/uc and the barrier ordering stays within the reported 0.07–0.19 eV window, the central ranking is robust; if the ordering changes or the barrier spread widens beyond the quoted range, the paper's claims about the hidden degree of freedom require substantial qualification.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claims are (i) MIL-120(Al)-A is the low-energy empty framework and (ii) CO2 location/energetics are governed by a hidden μ2-OH orientation degree of freedom. Both rest on the relative energies of six hand-built configurations, which differ by up to 0.59 eV/uc with interconversion barriers of 0.07–0.19 eV/uc. No functional sensitivity analysis is reported. The small barriers—and even the A-vs-F ordering—depend on PBE-D3's treatment of the interlocking hydrogen-bond network, a notoriously delicate quantity. PBE is known to under-bind H-bonds while D3 dispersion can over-stabilize certain compact arrangements; a 0.07–0.19 eV spread could plausibly reorder or widen under r2SCAN, SCAN-D3, or a hybrid functional. The text itself acknowledges that X-ray diffraction cannot locate H atoms (Sec. Introduction, Fig. 1c), so the A-configuration H-bond network is an unconfirmed computational prediction. Additionally, the six configurations are selected by hand, and the paper provides no enumeration, symmetry analysis, or machine-search evidence that these six exhaust the μ2-OH orientational landscape. If a seventh pattern existed or if the true ground-state pattern differed, the Qst match for A/B/C would be coincidental rather than diagnostic. Thus the single most load-bearing assumption is that PBE-D3 plus six hand-picked patterns faithfully represents the true energy landscape of μ2-OH orientations.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper combines PBE-D3 density-functional theory (DFT) with a purpose-trained DeePMD machine-learned potential (MLP) to investigate the orientations of bridging μ2-OH groups in the ultra-small pore MOF MIL-120(Al). Six manually constructed configurations (A–F) differing only in μ2-OH orientation are optimized, and the authors report that configuration A is the lowest-energy form while the commonly used F configuration is the highest, with an energy difference of about 0.59 eV per unit cell. CI-NEB calculations give interconversion barriers of 0.07–0.19 eV per unit cell, implying all six configurations are dynamically accessible at room temperature. The trained MLP is shown to reproduce DFT energies, barriers, phonon spectra, and CO2 adsorption geometries, and Widom insertion with the MLP yields isosteric heats of adsorption that match experiment for configurations A, B, and C. The central claim is that μ2-OH orientation is a 'hidden' structural degree of freedom controlling pore size and CO2 binding geometry (parallel vs. perpendicular), which has been overlooked in previous rigid-framework studies.","tokens_in":9429,"tokens_out":4549,"duration_ms":45393,"significance":"If the main conclusions hold, the paper provides a valuable correction to the standard structural model of MIL-120(Al) (F vs. A) and demonstrates that local hydroxyl dynamics can decisively control guest location and energetics in ultra-small pore MOFs. The work combines direct DFT energy calculations, transition-state searches, phonon analysis, and a large, openly available MLP training set (183,061 snapshots; data on GitHub), which is a reproducible and transparent computational resource. The identification of a hidden degree of freedom that is invisible to X-ray diffraction is conceptually important for the broader MOF adsorption community, where rigid-framework and fixed-hydrogen models are common. The MLP's near-DFT-level reproduction of energetics and geometries, while partly expected from the training set composition, still enables extensive sampling (Widom insertion, large-scale MD), and the direct DFT results provide a solid base for the central physics claim.","major_comments":[{"comment":"The relative energies of the six configurations and the 0.07–0.19 eV interconversion barriers are computed exclusively with PBE-D3 (Methods, 'DFT calculations'). The configurations differ chiefly by an interlocking hydrogen-bond network among μ2-OH groups, and PBE-D3 is known to be delicate for hydrogen-bond energetics: PBE tends to under-bind H-bonds while D3 dispersion can over-stabilize compact arrangements. Because the claim that A is the ground state and that all six states are thermally accessible rests directly on this energy ordering, the authors should test at least one additional functional (e.g., r2SCAN, SCAN-D3, or a hybrid) for the empty-framework relative energies and for the lowest-energy interconversion barrier, or provide a targeted justification why PBE-D3 is reliable for this specific H-bond network. Without such a check, the central energetic conclusion is not robust to functional choice.","section":"Methods (DFT calculations) and Results (Exploration)"},{"comment":"The MLP training set explicitly includes CI-NEB intermediate images, AIMD snapshots, and CO2-loaded configurations (Methods, 'Dataset preparation for MLP training'). Therefore, the reported MLP reproduction of the DFT CI-NEB barriers, phonon spectra, and CO2 adsorption geometries is an interpolation check on the training set rather than an independent prediction. The text states that the MLP 'quantitatively reproduced the DFT-derived barriers and transition-state geometries' and 'accurately predicting CO2 adsorption geometries', which overstates the evidential weight of these comparisons. The authors should explicitly acknowledge this circularity and, if possible, validate the MLP on a transition pathway or configuration held out from training (e.g., a leave-one-out CI-NEB path or a new AIMD trajectory at a temperature not sampled), or temper the predictive claims accordingly.","section":"Methods (Dataset preparation for MLP training)"},{"comment":"The six configurations are hand-selected 'representative' structures, and no enumeration, symmetry analysis, or systematic search is presented to show that they cover the full landscape of μ2-OH orientations. The paper later refers to 'all these states can be observed' and implies that the six configurations span the relevant structural space. If a seventh low-energy orientation pattern exists or the true ground-state pattern differs from A, the reported Qst match for A/B/C would be coincidental rather than diagnostic. The authors should provide a systematic search (e.g., enumeration of symmetry-inequivalent orientation patterns or Monte Carlo/temperature-accelerated sampling) to support the representativeness of the six configurations, or explicitly state in the Discussion that the six patterns are illustrative and not proven exhaustive.","section":"Results (Exploration of the local structural features)"}],"minor_comments":[{"comment":"The phrase 'throughout reorientation of functional groups' should read 'through reorientation of functional groups' (typo).","section":"Abstract"},{"comment":"The text refers to 'Supplementary Nate2'; this should be 'Supplementary Note 2'.","section":"Methods (DFT calculations)"},{"comment":"The MLP name is given as 'MACE-MPA-0' in the figure caption and 'MACE-MP-0' in the main text; the notation should be made consistent.","section":"Fig. 4b and main text"},{"comment":"The interaction-energy formula is corrupted in the extracted text (e.g., '𝐸!\"#=𝐸$%!@’%(-(𝐸’%+𝐸$%!)'); please provide a clean, standard equation for E_int.","section":"Fig. 4 caption"},{"comment":"The statement that the MLP achieves 'near-DFT-level fidelity' should be qualified, because the RMSE values (0.217 and 0.268 meV/atom) are computed on training or closely related configurations rather than on an independent test set; a brief clarification would help.","section":"Results (Machine-learned potential development)"},{"comment":"The phrase 'a large set of configurations' is used while only six were constructed; consider rewording to 'a set of six representative configurations' to avoid implying a more exhaustive sampling than was performed.","section":"Discussion"}],"recommendation":"major_revision","confidential_remarks":"The paper is a solid computational study for a specialized MOF, and the central physics claim is supported by direct DFT. However, the reliance on a single functional (PBE-D3) for the hydrogen-bond-dominated energy ordering, combined with the hand-selected configuration set, leaves the main quantitative conclusions vulnerable. The MLP circularity, while not fatal to the DFT-based conclusions, should be addressed because the paper's framing emphasizes MLP predictive power. These issues are fixable within the manuscript's scope but require additional calculations or clearly stated limitations."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague, read this one if you work on MOF/CO2 simulations. The new result: MIL-120(Al) has six low-barrier μ2-OH orientation states, the commonly used F model is not the ground state, and the OH orientation determines whether CO2 sits parallel or perpendicular to the channel. That is a real advance. Earlier work fixed a single H placement; this paper maps the landscape with direct DFT, not just MLP.\n\nThe direct DFT evidence is solid: six relaxed configurations, relative energies up to 0.59 eV/uc, CI-NEB barriers 0.07–0.19 eV/uc, phonons stable, and CO2 geometries computed at DFT level for all six. The Qst match for A/B/C against experiment is a nice consistency check. The MLP is well-made and the data/code are available, which I value.\n\nSoft spots, in proportion. The barrier 'reproduction' by the MLP is partly circular—CI-NEB images are in the training set. That is not fatal because the barriers are claimed from DFT too, but the headline near-DFT fidelity is an interpolation check, not a prediction. More important: the entire ranking of A–F rests on PBE-D3, and H-bond networks are exactly where PBE-D3 can mis-order small energy differences. The paper gives no second functional (r2SCAN, hybrid). Also, six hand-picked patterns is not a proof of exhaustive sampling. If a seventh pattern exists or the ordering changes under another functional, the specific ground state could shift; the broader conclusion that OH orientation matters would likely survive.\n\nWho this is for: anyone modeling CO2 capture in ultra-small-pore MOFs and anyone building MLPs for flexible MOFs. It deserves a serious referee; the main claims are empirically and computationally substantive. My recommendation: send to review, but ask the authors to add a functional sensitivity test and to state explicitly that the six patterns are representative, not exhaustive.","headline":"Solid computational study showing μ2-OH orientation is a hidden degree of freedom in MIL-120(Al); the PBE-D3-only energy ranking is the main caveat, not the MLP circularity.","tokens_in":10036,"tokens_out":2013,"would_cite":true,"duration_ms":20430,"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":"Hidden hydroxyl orientations set how CO2 binds in MIL-120(Al).","keywords":["metal-organic frameworks","MIL-120(Al)","CO2 capture","μ2-OH bridging hydroxyls","machine-learned potentials","configurational dynamics","adsorption energetics","density functional theory"],"falsifier":"A direct measurement of hydrogen positions by neutron powder diffraction or 2H solid-state NMR on MIL-120(Al) at room temperature would settle the ground-state ordering: if the dominant hydroxyl orientation is not the interlocking hydrogen-bond pattern of configuration A, or if hydroxyl flipping is not observed on experimental timescales, the claimed energy ranking and dynamic accessibility would be wrong.","tokens_in":8786,"feed_emoji":"💨","tokens_out":7842,"duration_ms":69473,"temperature":0.7,"pith_summary":"Using density-functional theory together with a purpose-trained machine-learned potential, this paper argues that the sorption behaviour of the ultra-small-pore MOF MIL-120(Al) is governed by a structural degree of freedom that standard diffraction cannot see: the orientation of the bridging μ2-OH hydroxyl groups. Six distinct framework configurations built from different hydroxyl orientations all sit close in energy, with interconversion barriers of 0.07–0.19 eV per unit cell, so several should be populated at room temperature. The paper identifies the configuration with an interlocking hydrogen-bond network, MIL-120(Al)-A, as the lowest-energy empty framework, contradicting the common practice of using the higher-energy MIL-120(Al)-F model. It further shows that hydroxyl orientation decides whether adsorbed CO2 lies parallel or perpendicular to the pore axis, and hence how strongly it binds. If correct, this means rigid or generic-force-field models that freeze hydroxyl positions will misplace guest molecules and misestimate adsorption energies in such MOFs.","feed_headline":"Hidden hydroxyl orientations set how CO2 binds in MIL-120(Al)","feed_subtitle":"Six hydroxyl states interconvert at room temperature and set where CO2 sits and how strongly it binds.","key_machinery":"The load-bearing object is the orientation pattern of the μ2-OH bridging hydroxyl groups in the MIL-120(Al) unit cell; each distinct pattern defines one framework configuration, and the paper treats this pattern as a hidden structural degree of freedom. The machine-learned potential carries the computation: trained on roughly 183,000 DFT-derived snapshots, including transition-path images, strained structures, and CO2-loaded configurations, it evaluates energies and forces with near-DFT accuracy, making it feasible to map the full interconversion landscape and to scan CO2 adsorption sites without the cost of direct DFT at every step. The hydrogen-bond network formed in the lowest-energy configuration A is the microscopic mechanism that stabilises that state, and the transient breaking and re-forming of these hydrogen bonds is the mechanism behind the low barriers.","core_discovery":"The central claim is that the local dynamics of the μ2-OH groups in MIL-120(Al) form a key, previously overlooked feature controlling CO2 capture. Concretely, the paper constructs six MIL-120(Al) models that differ only in the orientation of the four μ2-OH groups per unit cell. After DFT geometry optimization, MIL-120(Al)-A is the most stable and MIL-120(Al)-F, the model used in earlier work, is the least stable, with an energy gap of about 0.59 eV per unit cell. Transition-state calculations give interconversion barriers of 0.07–0.19 eV per unit cell, implying all six states are dynamically accessible at room temperature, and phonon calculations show each is dynamically stable. When CO2 is introduced, its molecular axis aligns perpendicular to the one-dimensional pore when μ2-OH groups point into the channel, and parallel when the hydroxyls are positioned more axially; this alignment difference is what sets the adsorption energetics. The paper's machine-learned potential reproduces the DFT energies, barriers, phonons, and CO2 adsorption geometries within a few kJ mol$^{-1}$, which is what allows the full hydroxyl configurational landscape to be mapped.","pith_inferences":["The same hidden-degree-of-freedom mechanism is likely at play in other ultra-small-pore MOFs whose pores are decorated with bridging hydroxyls, so structural models that fix hydrogen positions may need revisiting there too. A direct test would be low-temperature neutron diffraction or 2H NMR that can locate H atoms.","If configuration A really is the ground state, earlier computational studies that adopted F as the representative structure may have systematically biased their reported CO2 capacities and selectivities; re-running those calculations with A could change rankings among Al-MOF sorbents.","The low, guest-insensitive interconversion barriers suggest MIL-120(Al) could show measurable hydroxyl reorientation dynamics in variable-temperature infrared or inelastic neutron spectroscopy, which would be a direct experimental fingerprint of the proposed flipping.","The apparent success of a system-specific MLP in reproducing transition states hints that the strategy of training on CI-NEB images plus AIMD snapshots could be ported to other flexible MOFs with functional-group rotations, where generic MLIPs are weakest."],"forward_implications":["At room temperature, MIL-120(Al) should be treated as a dynamic mixture of at least six μ2-OH configurations; any single static structure is an idealisation.","The commonly used MIL-120(Al)-F model overestimates the isosteric heat of CO2 adsorption by about 27% relative to experiment, while the A, B, and C configurations match it.","CO2 adsorption geometry follows the local hydroxyl orientation: perpendicular alignment when OH points into the channel, parallel alignment when OH lies axial to the channel.","CO2 does not lock the framework: its presence changes the hydroxyl reorientation barriers by only a few percent, and in some pathways slightly lowers them.","Generic pre-trained machine-learned potentials can systematically over- or under-estimate CO2 interaction energies in this polar, flexible MOF; system-specific training is needed for reliable predictions."],"supporting_citations":[{"why":"Original synthesis and structure report of MIL-120(Al); supplies the experimental framework whose μ2-OH orientations are varied.","marker":"[16]"},{"why":"Reports the experimental CO2 adsorption performance and isosteric heat that the computed Qst values are compared against.","marker":"[17]"},{"why":"Demonstrates humid flue-gas CO2 capture on MIL-120(Al), the application context motivating the study.","marker":"[18]"},{"why":"Supplies the climbing-image nudged elastic band method used to compute interconversion barriers between configurations.","marker":"[19]"},{"why":"Provides the deep-learning potential framework used to build and train the machine-learned potential.","marker":"[20]"},{"why":"Shows the standard practice of adding hydrogen positions post hoc in computation-ready MOF structures, the assumption the paper challenges.","marker":"[15]"},{"why":"A pre-trained generic machine-learning force field used as a baseline for CO2 interaction energies.","marker":"[26]"},{"why":"A transferable machine-learning force field for direct air capture screening, used as a second baseline.","marker":"[27]"}],"fun_headline_variants":["Hydroxyl flips dictate CO2 capture in MIL-120(Al)","Six hydroxyl states control CO2 adsorption in MIL-120(Al)","Machine learning maps hydroxyl dynamics for CO2 capture","Local hydroxyl orientation sets CO2 binding in MIL-120(Al)"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The energy ordering and the 0.07–0.19 eV barrier window rest on a single density-functional approximation for hydrogen-bond strengths and on the assumption that the six hand-selected μ2-OH orientation patterns are representative, an assumption X-ray diffraction cannot check because it does not locate hydrogen atoms.","fun_headline_variants_meta":{"raw":{"variants":["Hydroxyl flips dictate CO2 capture in MIL-120(Al)","Six hydroxyl states control CO2 adsorption in MIL-120(Al)","Machine learning maps hydroxyl dynamics for CO2 capture","Local hydroxyl orientation sets CO2 binding in MIL-120(Al)"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000179,"raw_usage":{"total_tokens":1366,"prompt_tokens":1076,"completion_tokens":290,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":692,"completion_tokens_details":{"reasoning_tokens":217}},"tokens_in":692,"tokens_out":290,"duration_ms":2968,"temperature":1.0,"reasoning_tokens":217,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T16:42:17.958201+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A direct measurement of hydrogen positions by neutron powder diffraction or 2H solid-state NMR on MIL-120(Al) at room temperature would settle the ground-state ordering: if the dominant hydroxyl orientation is not the interlocking hydrogen-bond pattern of configuration A, or if hydroxyl flipping is not observed on experimental timescales, the claimed energy ranking and dynamic accessibility would be wrong.","supporting_citations":[],"review_version":2}