REVIEW 3 major objections 6 minor 22 references
Force-Isosurface Simulations Probe the Limits of High-Resolution AFM on Three-Dimensional Molecules
T0 review · 3 major / 6 minor · reviewed 2026-07-09 · glm-5.2
Pith's one-line read Constant-force AFM recovers 3D molecular structure lost in conventional imaging
desk verdict Solid simulation study with a real but unevenly distributed electrostatics gap; deserves a serious referee read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The Probe Particle Model simulates a flexible CO-like tip apex interacting with molecular coordinates through Lennard–Jones potentials. A 3D force field is computed on a grid above each molecule. A custom extraction algorithm searches each vertical column from large tip–sample separation toward the molecule and records the highest z-position where the force threshold (typically 250 pN) is exceeded, producing the outermost force isosurface. Plane fitting to selected regions of this isosurface recovers molecular tilt angles.
What would settle it
If adding electrostatic interactions to the probe-particle-model simulations substantially changes the shape of the force isosurface for pyrrole, ZnTPP, or CO-FePc — for instance by shifting the contour above polar regions enough to obscure the lower-lying features the paper claims are revealed — then the predicted contrasts and tilt-angle recovery would not hold for these molecules in real experiments.
Extended reading notes
Core claim
The central object is the force isosurface: the height contour defined not by a fixed plane but by a constant force threshold in the 3D tip–sample interaction field. The paper demonstrates that this isosurface retains submolecular and three-dimensional structural information — molecular rings, tilt angles, central metal atoms, lower-lying cores beneath protruding ligands, and adjacent faces on curved molecules — that is progressively lost in constant-height images as molecular non-planarity increases. For simple tilted fragments the isosurface recovers orientation to within 1 degree; for complex molecules it distinguishes adsorption geometries and reveals features otherwise hidden, though it
Load-bearing premise
The simulations include only Lennard–Jones (van der Waals and Pauli repulsion) interactions and omit electrostatic forces entirely. Several molecules studied — pyrrole with its polar N–H bond, ZnTPP with a central metal ion, and CO-FePc with a CO ligand carrying a significant dipole — have electrostatic contributions that could reshape the force isosurface and alter the specific contrasts and quantitative angle recovery reported.
Editorial extensions
If this is right
- Experimental 3D force-mapping protocols can use these simulated isosurfaces as reference templates to verify that measured force contours contain genuine structural information rather than artefacts of tip relaxation or feedback instability.
- Force-feedback AFM modes (PeakForce, off-resonance) that already acquire force curves during imaging could extract isosurface contrast directly, potentially bringing 3D molecular structure determination to ambient or room-temperature conditions without requiring full UHV frequency-shift tomography.
- For molecules with protruding functional groups — axial ligands, metal centres, or adsorbed species — force-isosurface imaging could simultaneously resolve the protrusion and the underlying molecular framework, which is structurally important for understanding catalytic sites or on-surface reaction intermediates.
- The systematic underestimation of tilt angles in the saddle geometry of 2H-TPP demonstrates that neighbouring molecular features distort the local isosurface, which means quantitative structural recovery in complex molecules will require simulation-guided interpretation rather than direct geometric reading of the contour.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This manuscript uses Probe Particle Model (PPM) simulations to extract force isosurfaces from 3D force fields above molecules of increasing structural complexity, from tilted benzene and pyrrole to 2H-TPP, ZnTPP, CO-FePc, and C60. The central claim is that constant-force (force-isosurface) imaging retains submolecular and 3D structural information that is lost in constant-height imaging of non-planar molecules. For tilted benzene and pyrrole, the authors show quantitative recovery of tilt angles to within 1 degree. For larger systems, they identify characteristic contrast patterns associated with different adsorption geometries, central metal features, and curved molecular surfaces. The progression from simple to complex systems is well-designed, and the honest reporting of the 5-degree systematic underestimation for the 2H-TPP saddle geometry is commendable.
Significance. The paper provides falsifiable, molecule-specific target contrasts for force-isosurface imaging that could guide future 3D force-mapping experiments. The quantitative tilt-angle recovery for benzene and pyrrole (Figure 3, within 1 degree) is a clean validation result. The systematic comparison of constant-height versus constant-force imaging across a range of molecular complexities is a useful contribution to the field. The simulation methodology is standard and sound, and the parameter choices (250 pN threshold, 4 N/m stiffness, CO-like probe) are drawn from established PPM practice. The paper does not ship reproducible code or machine-checked proofs, but the methodology is sufficiently described for replication.
major comments (3)
- §'Simulating constant-force images': The omission of electrostatic interactions is stated but its implications are not analyzed. This is a load-bearing simplification for the ZnTPP (Figure 5) and CO-FePc (Figure 6) results, where the distinguishing features involve charged or polar species (Zn2+ center, CO ligand dipole). At the ~2.5-3 Å probe-sample separations corresponding to 250 pN repulsion, electrostatic forces from a CO-terminated tip interacting with a +1e metal center are plausibly on the order of 50-200 pN, which is not negligible relative to the 250 pN threshold. The paper should either (a) provide a sensitivity analysis or order-of-magnitude bound on how electrostatics would modify the predicted contrast for ZnTPP and CO-FePc, or (b) explicitly scope the claims for these two systems as purely LJ-based predictions that may not hold quantitatively in experiment. The benzene/pyr
- §'Differentiating between adsorption geometries of 2H-TPP', Figure 3c,d: The 5-degree systematic underestimation of the phenyl-ring angle for the saddle geometry is attributed to 'the influence of the surrounding molecular structure on the local force isosurface.' This attribution is plausible but not demonstrated. A simple test would be to fit the plane to a smaller or shifted region of the phenyl-ring isosurface to check whether the shoulder from the macrocycle is the actual cause, or whether the discrepancy arises from the plane-fitting procedure itself. Without this, the reader cannot assess whether the 5-degree error is a fundamental limitation of force-isosurface imaging for crowded molecules or an artifact of the fitting protocol. This matters because the paper's central quantitative claim — that molecular orientation can be recovered from force isosurfaces — is directly tested by
- §'Metal-centred porphyrins and phthalocyanines', Figure 5: The claim that the Zn atom 'produces a raised central feature' with 'contrast extending towards the four surrounding nitrogen atoms' is presented without comparison to a 2H-TPP constant-force image at the same setpoint. The text references Albrecht et al. [12] showing that CuTPP produces a filled centre compared to 2H-TPP, but the reader cannot visually confirm this distinction from the figures provided, since Figure 4 (2H-TPP) and Figure 5 (ZnTPP) use different colour scales and height ranges. A direct side-by-side comparison with matched scales would substantially strengthen the claim that the central feature is attributable to the metal atom rather than to the saddle geometry itself.
minor comments (6)
- Figure 2: The line profiles are labelled with force values in pN but the relationship between the profile positions and the molecular structure is not always clear. Adding molecular skeleton overlays or tick marks at atom positions would help readers interpret the profiles.
- Figure 3c,d: The 2H-TPP data points are plotted alongside benzene and pyrrole but use different marker shapes (diamonds vs hexagons/pentagons). A legend entry or caption note clarifying which diamond colour corresponds to which geometry would help readers interpret the plot.
- §'Differentiating between adsorption geometries of 2H-TPP': The phenyl-ring angles for the inverted geometry are reported as 3.9, 3.9, 3.8, and 3.9 degrees with error ±0.5 degrees. It is unclear whether this error is a fitting uncertainty, an RMS residual, or something else. This should be specified.
- §'Metal-centred porphyrins and phthalocyanines': The C60 colour scales in Figure 7 are described as 'focused on the uppermost face' and 'adjusted to lower heights,' but the specific height ranges used for each panel are not stated. This should be specified.
- The paper would benefit from a brief discussion of the experimental accessibility of the simulated force thresholds. The 250 pN setpoint is within the range of PeakForce-style methods, but it would help to note whether such thresholds are achievable under typical UHV NC-AFM conditions.
- Reference [18] is cited for both the CO-FePc experimental work (Chen et al.) and the PPM methodology description. These appear to be different works and should be disambiguated.
Simulated Author's Rebuttal
We thank the referee for a careful and constructive report. The referee correctly identifies the main contributions of the paper and raises three substantive points regarding (1) the omission of electrostatic interactions, (2) the untested origin of the 5-degree underestimation in the 2H-TPP saddle geometry, and (3) the lack of a matched-scale comparison between ZnTPP and 2H-TPP. We address each point below.
read point-by-point responses
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Referee: Omission of electrostatic interactions not analyzed, especially for ZnTPP and CO-FePc where charged/polar species are involved.
Authors: The referee is correct that electrostatic forces are not negligible at the probe-sample separations corresponding to 250 pN repulsion, and we should have been more explicit about the scope and limitations this introduces. We agree with option (b): the claims for ZnTPP and CO-FePc should be explicitly scoped as Lennard-Jones-based predictions. We will add a paragraph to the methodology section stating that electrostatic contributions from the Zn2+ centre and CO ligand dipole are expected to modify the predicted contrast at the ~2.5-3 Å separations involved, and that the ZnTPP and CO-FePc results should be interpreted as purely LJ-based predictions that identify which structural features are expected to dominate the short-range force contrast. We will also add an order-of-magnitude estimate of the electrostatic force from a CO-terminated tip interacting with a +1e metal centre at these separations to make clear that this is a non-negligible effect. The benzene, pyrrole, 2H-TPP, and C60 results, which do not involve charged or strongly polar species, are less affected by this omission. revision: yes
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Referee: The 5-degree underestimation for 2H-TPP saddle geometry is attributed to surrounding molecular structure but not demonstrated; a test fitting to a smaller or shifted region is needed.
Authors: The referee makes a fair point: our attribution of the 5-degree underestimation to the shoulder from the macrocycle is plausible but untested. We will perform the suggested test by fitting the plane to smaller and shifted regions of the phenyl-ring isosurface, specifically excluding the region where the macrocycle shoulder is expected to contribute. This will allow us to determine whether the discrepancy is reduced when the neighbouring structure is excluded, or whether it arises from the plane-fitting procedure itself. We will report the results of this test in the revised manuscript, either confirming or revising our attribution. If the discrepancy persists with reduced fitting regions, we will reframe the discussion to acknowledge that the origin is not fully established. revision: yes
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Referee: ZnTPP central feature claim lacks a side-by-side comparison with 2H-TPP at the same setpoint and matched colour scales.
Authors: This is a reasonable request and we will implement it. We will produce a side-by-side comparison of the 2H-TPP and ZnTPP constant-force images at the same 250 pN setpoint with matched colour scales and height ranges, so that the reader can directly assess whether the central raised feature in ZnTPP is attributable to the metal atom rather than to the saddle geometry itself. This comparison will be added as a new figure or panel in the revised manuscript. revision: yes
Circularity Check
No significant circularity found; the paper is a forward-looking simulation study with transparent validation methodology.
full rationale
The paper simulates force isosurfaces using the Probe Particle Model (PPM, cited to external authors Hapala et al. [6]) with parameters (250 pN, 4 N/m) selected from a parameter scan (Figure S1), not fitted to target results. The tilt-angle recovery for benzene and pyrrole (Figure 3) is an explicit validation test: known tilt angles are inputs to the simulation, and the paper checks whether the isosurface preserves them. This is standard methodology, not circular reasoning. The 2H-TPP coordinates come from self-citation [13] (Jarvis et al., 2015), but these are structural inputs — the force isosurfaces are computed from them via PPM and are not equivalent to the coordinates by construction. The Sader-Jarvis formalism [22] is mentioned only as an experimental force-conversion method and is not load-bearing for any simulation result. The CO-FePc coordinates come from external authors [18]. No step in the derivation chain reduces to its own inputs by definition, no uniqueness theorem is invoked, and no ansatz is smuggled through self-citation. The one minor self-citation [13] provides independent structural data and does not raise circularity concerns.
Assumptions & free parameters
free parameters (3)
- Force threshold =
250 pN
- Lateral probe-particle stiffness =
4 N/m
- Probe particle type =
CO-like
assumptions (3)
- domain assumption Lennard-Jones potentials alone adequately represent the short-range tip-sample force for the purpose of comparing constant-height and constant-force imaging contrast.
- domain assumption The Probe Particle Model with a CO-like probe and 4 N/m stiffness accurately reproduces the force fields above these molecules.
- domain assumption The force isosurface extraction algorithm (searching from largest separation toward the molecule for the highest z where the force threshold is exceeded) produces a unique and physically meaningful surface.
Cite this review
Pith. "Pith review of Force-Isosurface Simulations Probe the Limits of High-Resolution AFM on Three-Dimensional Molecules." pith.science (2026). https://pith.science/paper/DRUKFASS
@misc{pith2026260707363,
author = {Pith},
title = {Pith review of: Force-Isosurface Simulations Probe the Limits of High-Resolution AFM on Three-Dimensional Molecules},
year = {2026},
howpublished = {\url{https://pith.science/paper/DRUKFASS}},
note = {Machine review of arXiv:2607.07363}
}
read the original abstract
High-resolution atomic force microscopy has transformed molecular imaging by revealing intramolecular structure directly in real space. A major remaining challenge is to extend this capability from largely planar molecules to non-planar molecular systems, where the most important structural information may be distributed across different heights above the surface. Here we use probe-particle-model simulations to predict the constant-force contours expected above molecules with increasing structural complexity. By extracting force isosurfaces from simulated three-dimensional force fields, we compare the molecular information retained in constant-height and constant-force images. For tilted benzene and pyrrole, constant-force images preserve the molecular framework across a range of adsorption angles and allow the molecular orientation to be recovered quantitatively. For larger non-planar and three-dimensional systems, simulations identify characteristic force-isosurface contrast associated with adsorption geometry, lower-lying molecular structure and curved molecular surfaces. These results provide target contrasts for force isosurfaces that could be extracted from three-dimensional force-mapping experiments, evaluating the molecular information retained by ideal force-isosurface imaging across progressively non-planar systems.
Figures
Figures from the paper (4 more)
Reference graph
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