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arxiv: 2606.07823 · v1 · pith:CGQH3RPZ · submitted 2026-06-05 · cond-mat.soft

Exact mean-field phase diagram for self-avoiding active particles in a lattice

Felipe Hawthorne , Cristiano F. Woellner , Jos\'e A. Freire This is my paper

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classification cond-mat.soft
keywords motility-induced phase separationmean-field theorylattice gasactive particlesspinodal surfaceBravais latticestight-binding modelbroken detailed balance
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0 comments X

The pith

Mean-field master equation yields closed analytical spinodal surfaces for active particles on six Bravais lattices.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes an exact analytical expression for the spinodal surface that marks the linear instability of the uniform state in a lattice gas of self-propelled particles subject to hard-core exclusion, directed hopping, rotational diffusion, and translational diffusion. It proceeds by linearizing the mean-field master equation, invoking Bloch's theorem to reduce the problem to a z-dimensional tight-binding eigenvalue problem, and performing a small-wavenumber perturbation expansion that isolates all lattice dependence into a single coefficient A. A reader would care because the resulting formulas give the critical density as an explicit function of activity, diffusion rates, and lattice type without requiring stochastic simulations, while also showing that translational diffusion smooths phase interfaces and that rotational currents persist in the inhomogeneous states.

Core claim

Linearization of the mean-field master equation around the homogeneous stationary state, followed by Bloch's theorem, reduces the stability analysis to a z-dimensional tight-binding eigenvalue problem. A perturbation expansion in wavenumber near zero then produces the spinodal surface in closed form for the linear, square, hexagonal, simple cubic, body-centered cubic, and face-centered cubic lattices, with all geometric influence captured by one coefficient A that is evaluated exactly in each case. Translational diffusion is shown to smooth the interface between dense and dilute phases, and the rotational probability currents associated with the inhomogeneous states are computed explicitly.

What carries the argument

The z-dimensional tight-binding eigenvalue problem obtained after linearizing the mean-field master equation and applying Bloch's theorem, whose small-wavenumber expansion isolates lattice geometry into the single coefficient A.

If this is right

  • The spinodal density is obtained as an explicit algebraic function of the model parameters for each lattice.
  • All effects of lattice geometry are confined to the single exactly computed coefficient A.
  • Translational diffusion reduces the sharpness of the dense-dilute interface.
  • Inhomogeneous stationary states carry nonzero rotational probability currents that signal broken detailed balance.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same reduction to a tight-binding problem could be applied to lattices with longer-range hops or to continuous-space limits.
  • The explicit form of A for each lattice supplies a quantitative ranking of how strongly geometry stabilizes or destabilizes the uniform phase.
  • The rotational currents computed for the phase-separated states offer a measurable signature that distinguishes active from passive phase separation in experiments.

Load-bearing premise

The general mean-field approximation to the master equation is sufficient to locate the linear instability of the homogeneous state.

What would settle it

Numerical comparison of the analytically predicted spinodal densities against direct stochastic simulations of the underlying master equation on the same six lattices.

Figures

Figures reproduced from arXiv: 2606.07823 by Cristiano F. Woellner, Felipe Hawthorne, Jos\'e A. Freire.

Figure 1
Figure 1. Figure 1: FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: illustrates two different inhomogeneous sta￾tionary solutions of the mean field master equation for a 20 × 20 square lattice, both obtained with the same set of model parameters where the homogeneous solu￾tion is unstable (checked numerically), i.e., within the spinodal region. We simply integrated Eq.(3) until con￾vergence, starting from two different initial conditions. The average site occupation, ∑ s p… view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: illustrates the probability current of the two inhomogeneous stationary states in [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: D. The Metastable Region The metastable region is the portion of parameter space outside the spinodal surface and inside the binodal surface in which the homogeneous state remains linearly stable yet coexists with locally stable inhomogeneous sta￾tionary states. If an underlying free-energy function ex￾isted, this region would be characterized by multiple free￾energy minima. Outside the binodal surface the… view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6 [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 1
Figure 1. Figure 1: illustrates the fluctuation spectrum in the case of a square lattice, whose first Brillouin zone (1BZ) is displayed in [PITH_FULL_IMAGE:figures/full_fig_p009_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p012_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6 [PITH_FULL_IMAGE:figures/full_fig_p013_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7 [PITH_FULL_IMAGE:figures/full_fig_p014_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8 [PITH_FULL_IMAGE:figures/full_fig_p016_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9 [PITH_FULL_IMAGE:figures/full_fig_p018_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10 [PITH_FULL_IMAGE:figures/full_fig_p018_10.png] view at source ↗
read the original abstract

We investigate motility-induced phase separation in a lattice gas of self-propelled particles with hard-core exclusion, where an internal director biases particle hopping along the lattice coordination directions while undergoing rotational diffusion, together with a thermal-like translational diffusion. Rather than employing stochastic simulations, we adopt a master-equation formalism within a general mean-field approximation. By linearizing the mean-field master equation around the homogeneous stationary state and applying Bloch's theorem, the stability analysis is reduced to a $z$-dimensional tight-binding eigenvalue problem. A perturbation expansion in the wavenumber near $\vk = 0$ then yields the spinodal surface in closed analytical form for six Bravais lattices: linear, square, hexagonal, simple cubic, body-centered cubic, and face-centered cubic. The influence of lattice geometry is shown to enter exclusively through a single coefficient $\mathcal{A}$ which we evaluate exactly for each case. We further show that translational diffusion smooths the interface between the dense and dilute phases. Finally, we determine the rotational probability currents associated with the inhomogeneous stationary states, a distinctive signature of the broken detailed balance underlying active-system dynamics.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 2 minor

Summary. The manuscript develops a mean-field theory for motility-induced phase separation in a lattice gas of self-propelled particles subject to hard-core exclusion, internal director bias, rotational diffusion, and translational diffusion. Starting from the master equation under a general mean-field closure, the homogeneous state is linearized; Bloch's theorem reduces the stability problem to a z-dimensional tight-binding eigenvalue problem whose small-k expansion yields a closed-form spinodal surface. Lattice geometry enters solely through a single exactly evaluated coefficient A for the linear, square, hexagonal, simple-cubic, bcc and fcc lattices. Translational diffusion is shown to smooth interfaces and rotational probability currents are computed for the inhomogeneous states.

Significance. If the derivation holds, the work supplies parameter-free, analytically closed spinodal expressions across six Bravais lattices, a clean reduction of all geometric dependence to the single scalar A, and explicit non-equilibrium currents. These features constitute a reproducible, falsifiable benchmark for lattice active-matter models and enable direct comparison with simulations without fitting parameters.

minor comments (2)
  1. [Abstract] The abstract states that the spinodal is obtained 'in closed analytical form'; a brief remark in the main text confirming that the final expression for the critical density contains no implicit numerical roots would strengthen this claim.
  2. A compact table listing the exact numerical values of A for each of the six lattices would improve readability and allow immediate cross-lattice comparison.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of the manuscript and for recommending acceptance. The referee's summary accurately reflects the derivation of the exact mean-field spinodal surfaces via the Bloch reduction and the role of the single coefficient A across the six lattices.

Circularity Check

0 steps flagged

No significant circularity identified

full rationale

The paper derives the spinodal surface via direct linearization of its stated mean-field master equation, followed by Bloch's theorem reduction to a tight-binding eigenvalue problem and a small-k perturbation expansion. These steps are algebraic consequences of the model's own equations and standard lattice symmetry arguments; lattice dependence collapses to a single exactly computed coefficient A with no fitted inputs, no self-citations invoked as load-bearing premises, and no renaming or ansatz smuggling. The derivation is therefore self-contained against the paper's own closure and does not reduce to its inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The derivation rests on the mean-field closure of the master equation and the assumption that the linear instability coincides with the spinodal; no free parameters are introduced beyond the physical rates already present in the model description.

axioms (2)
  • domain assumption Mean-field approximation closes the master equation by factoring joint probabilities into single-particle densities.
    Invoked when the stability analysis is performed on the mean-field master equation (abstract).
  • domain assumption Linearization around the homogeneous stationary state identifies the onset of phase separation.
    Used to reduce the problem to the eigenvalue problem whose zero mode signals the spinodal.

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Reference graph

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