{"id":"f4e09c09-efd9-4d60-ac9b-3b486e18cf84","arxiv_id":"2606.27133","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":5.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":3,"one_line_summary":"Internal friction between coarse-grained beads is required to reproduce globular protein dynamics in water, with parameters optimized via self-averaging to match all-atom MD statistics.","lead":"The paper shows that coarse-grained models of globular proteins require an internal friction term between beads, beyond elastic couplings and solvent self-friction, to match all-atom molecular dynamics. A self-averaging optimization integrates the CG simulation with parameter evolution to match radial distribution functions and bead velocity correlations.","discovery_kind":"new_method","skeptic_critique":{"model":"grok-4.3","headline":"Matching only RDF and bead-velocity autocorrelation may not guarantee faithful capture of full protein dynamics","rationale":"The reader's weakest_assumption is precisely the load-bearing step in the argument. No stronger internal inconsistency is visible from the supplied abstract; the proposed test directly probes whether the chosen matching targets are adequate for the broader claim.","tokens_in":1743,"tokens_out":296,"duration_ms":11083,"concrete_test":"From the final optimized CG trajectories, compute the long-time self-diffusion coefficient of the protein center of mass and the autocorrelation decay time of the radius of gyration; compare both directly to the corresponding all-atom MD values. A discrepancy larger than the statistical uncertainty while the two optimized quantities remain matched would show that the matching criterion is insufficient.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim requires that internal friction is critically necessary for faithful dynamics reproduction. The method optimizes CG parameters (elastic couplings, self-friction, internal friction) via a self-averaging procedure so that the radial distribution function and the time correlation of bead velocities match all-atom MD. These two observables constrain static structure and short-time dissipative response but leave unconstrained longer-time transport (e.g., center-of-mass diffusion), conformational relaxation spectra, or cross-correlations between distant beads. If the internal-friction term improves the targeted matches yet leaves other dynamical signatures discrepant, the necessity claim for the full dynamics does not follow.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The manuscript develops a coarse-grained (CG) protein model via the Mori-Zwanzig formalism and claims that, in addition to elastic couplings and Stokesian self-friction, internal friction between beads is critically necessary to reproduce the dynamics of a globular protein in water. This necessity is demonstrated by comparing all-atom MD reference data with CG simulations whose parameters (including internal-friction coefficients) are optimized through a self-averaging procedure so that the radial distribution function and bead-velocity time correlations match the MD values.","tokens_in":1902,"tokens_out":491,"duration_ms":41848,"significance":"If the necessity claim can be shown to hold for observables beyond the two fitted quantities, the work would provide a concrete route to incorporate missing dissipative effects from unresolved degrees of freedom into CG biomolecular models, moving beyond purely hydrodynamic dissipation.","major_comments":[{"comment":"Abstract: the claim that internal friction is 'critically necessary for a faithful representation of protein dynamics' is supported only by improved matching of the two quantities (RDF and velocity autocorrelation) to which the internal-friction coefficients are explicitly fitted; because agreement on these observables is achieved by construction, an independent test on at least one unfitted dynamical signature (e.g., center-of-mass diffusion or conformational relaxation spectrum) is required to substantiate the necessity assertion.","section":"Abstract"},{"comment":"Self-averaging optimization procedure: the evolution equation for the CG parameters (elastic constants, self-friction, and internal-friction matrix) is driven to reproduce MD-derived RDF and velocity correlations; this leaves unconstrained longer-time transport and cross-correlations, so the manuscript must show that the internal-friction term improves predictions for these additional properties rather than merely compensating for the limited constraint set.","section":"Methods (self-averaging procedure)"}],"minor_comments":[{"comment":"The precise functional form of the internal-friction term (e.g., whether it is a pairwise distance-dependent matrix or a constant) and its relation to the Mori-Zwanzig memory kernel should be stated with an explicit equation to allow reproducibility.","section":null}],"recommendation":"major_revision","confidential_remarks":"The central methodological concern is that the validation remains internal to the fitted observables; this is a load-bearing issue for the necessity claim and should be resolved before acceptance."},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for the constructive comments. We agree that the necessity of internal friction requires demonstration on observables beyond the fitted RDF and velocity correlations, and we will revise the manuscript to include such independent tests.","responses":[{"response":"We acknowledge that the current support for the necessity claim rests on the two quantities used in the self-averaging optimization. In the revised manuscript we will add direct comparisons, for models with and without the internal-friction matrix, of the center-of-mass diffusion coefficient and the conformational relaxation spectrum against the all-atom MD reference. These unfitted observables will serve as an independent validation.","revision_made":"yes","referee_comment":"[Abstract] Abstract: the claim that internal friction is 'critically necessary for a faithful representation of protein dynamics' is supported only by improved matching of the two quantities (RDF and velocity autocorrelation) to which the internal-friction coefficients are explicitly fitted; because agreement on these observables is achieved by construction, an independent test on at least one unfitted dynamical signature (e.g., center-of-mass diffusion or conformational relaxation spectrum) is required to substantiate the necessity assertion."},{"response":"We agree that the optimization is performed on a limited set of statistics. The revised version will therefore report the performance of the optimized models (with versus without internal friction) on longer-time transport coefficients and cross-correlations that are not part of the fitting target, thereby testing whether the internal-friction term captures additional dissipative physics.","revision_made":"yes","referee_comment":"[Methods (self-averaging procedure)] Self-averaging optimization procedure: the evolution equation for the CG parameters (elastic constants, self-friction, and internal-friction matrix) is driven to reproduce MD-derived RDF and velocity correlations; this leaves unconstrained longer-time transport and cross-correlations, so the manuscript must show that the internal-friction term improves predictions for these additional properties rather than merely compensating for the limited constraint set."}],"tokens_in":1391,"tokens_out":427,"duration_ms":52604,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The paper's core point is that coarse-grained models of globular proteins need an extra internal friction term between beads, on top of elastic couplings and solvent self-friction, to get the dynamics right when compared to all-atom MD. They derive this from Mori-Zwanzig and then optimize the CG parameters with a self-averaging scheme so the radial distribution function and bead velocity time correlations line up with the atomistic data.\n\nWhat works is the direct comparison: without the internal friction the velocity correlations deviate, and adding it improves the match. Treating the missing degrees of freedom as a source of friction inside the protein is a reasonable extension of existing polymer ideas, and the optimization method gives a concrete way to set the parameters.\n\nThe limitation is that the agreement is achieved by construction. The parameters are adjusted until those two observables match, so we do not see an independent test of whether the model then gets other dynamical features correct, such as center-of-mass diffusion or longer conformational relaxation times. The abstract's claim that internal friction is critically necessary for faithful protein dynamics therefore rests on a narrow set of targets. The stress-test concern about unconstrained longer-time transport holds up on the evidence given.\n\nThis is for people who build or use CG models for biomolecules and want the dissipative part handled more carefully. It is worth sending to peer review because the comparison is concrete and the modeling choice addresses a real gap, even if additional checks would make the necessity argument stronger.","headline":"The paper shows internal friction helps match velocity correlations in this CG protein model, but the fitting to just RDF and autocorrelations limits how far the necessity claim travels.","tokens_in":2385,"tokens_out":372,"would_cite":false,"duration_ms":23607,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"Coarse-grained protein models require internal friction between beads to accurately reproduce their dynamics in water.","keywords":["coarse-graining","protein dynamics","internal friction","Mori-Zwanzig formalism","hydrodynamic interactions","molecular dynamics"],"falsifier":"A side-by-side comparison in which a coarse-grained model that includes the fitted internal friction is tested against all-atom results on an observable not used in the fitting procedure, such as the long-time mean-squared displacement of the protein center of mass or its rotational correlation time.","tokens_in":2659,"feed_emoji":"🧬","tokens_out":655,"duration_ms":46150,"temperature":0.7,"pith_summary":"The paper shows that coarse-grained models of globular proteins in water must include internal friction between beads, which arises from the atomic motions removed during coarse-graining. Elastic forces and solvent hydrodynamic friction alone do not suffice to match the time-dependent behavior observed in all-atom simulations. The authors compare the coarse-grained trajectories directly to molecular dynamics references and introduce a self-averaging optimization that evolves model parameters until the radial distribution function and bead velocity correlations agree with the atomistic data.","feed_headline":"Coarse-grained proteins need internal friction to match dynamics","feed_subtitle":"Hydrodynamic couplings alone miss dissipation from hidden atomic motions inside each bead","key_machinery":"The internal friction term in the dissipative bead-bead interactions, arising from unresolved internal degrees of freedom via the Mori-Zwanzig projection.","core_discovery":"In order to accurately reproduce the dynamics of a globular protein in water using a coarse-grained model, not only a precise determination of elastic couplings and the Stokesian self-friction of each bead is required. Critically, the inclusion of internal friction between beads is also necessary for a faithful representation of protein dynamics. Parameters are optimized by a self-averaging method that integrates the coarse-grained dynamics with an evolution equation until the radial distribution function and the time correlation of bead velocities match the corresponding all-atom values.","pith_inferences":["Similar internal friction contributions are likely required when coarse-graining other macromolecules whose internal motions are integrated out.","The method could be tested on non-equilibrium driving, where internal friction would alter the rate of energy dissipation in steered simulations.","If the claim holds, existing hydrodynamic coarse-grained force fields will need reparameterization before they can predict folding pathways or ligand binding kinetics."],"forward_implications":["Coarse-grained trajectories without internal friction will produce incorrect relaxation rates and diffusion behavior even when structure is matched.","The self-averaging optimization supplies a consistent route to determine the full set of dissipative parameters including internal friction.","Protein simulations on timescales inaccessible to all-atom methods become reliable once internal friction is incorporated.","Standard hydrodynamic-only treatments of bead interactions systematically omit a source of dissipation that affects all time correlations."],"fun_headline_variants":["Internal bead friction fixes coarse-grained protein dynamics","Coarse-grained models need internal friction for protein dynamics","Internal friction between beads required in CG protein models","Accurate dynamics demand internal friction in coarse-grained beads"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"Matching only the radial distribution function and the time correlation of bead velocities is sufficient to guarantee that the coarse-grained model with added internal friction faithfully captures the full protein dynamics.","fun_headline_variants_meta":{"raw":{"variants":["Internal bead friction fixes coarse-grained protein dynamics","Coarse-grained models need internal friction for protein dynamics","Internal friction between beads required in CG protein models","Accurate dynamics demand internal friction in coarse-grained beads"]},"model":"grok-4.3","cost_usd":0.00484,"raw_usage":{"total_tokens":2389,"prompt_tokens":691,"num_sources_used":0,"completion_tokens":57,"cost_in_usd_ticks":48399500,"prompt_tokens_details":{"text_tokens":691,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":1641,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":691,"tokens_out":57,"duration_ms":25041,"temperature":1.0,"reasoning_tokens":1641,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-06-26T02:29:14.088370+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"A side-by-side comparison in which a coarse-grained model that includes the fitted internal friction is tested against all-atom results on an observable not used in the fitting procedure, such as the long-time mean-squared displacement of the protein center of mass or its rotational correlation time.","supporting_citations":[],"review_version":1}