- Article
Compact Analytic Two-Gaussian Representation of Universal Short-Range Coulomb Correlations in Soft-Core Fluids
- Hiroshi Frusawa
Soft-core Coulomb fluids, exemplified by the two-dimensional Gaussian-charge one-component plasma, serve as fundamental benchmarks for both mathematical theory and computational modeling of coarse-grained dynamics, including stochastic density functional theory, dynamical density functional theory, and dissipative particle dynamics. In these systems, the conventional mean-field description, or the random phase approximation (RPA), is frequently employed due to its analytic simplicity; however, its validity is restricted to weak coupling regimes. Here we demonstrate that Coulomb correlations induce a structural crossover to a strongly correlated liquid where the nearest-neighbor distance saturates rather than decreasing monotonically, a behavior fundamentally incompatible with mean-field predictions. Central to our analysis is the emergence of a universal scaling law: when rescaled by the coupling constant, the short-range direct correlation function (DCF) collapses onto a single curve across the strong coupling regime. Exploiting this universality, we construct a closed-form analytic representation of the DCF using a two-Gaussian basis. This compact form accurately reproduces hypernetted-chain radial distribution functions and structure factors while ensuring exact compliance with thermodynamic sum rules. Beyond theoretical elegance, the proposed kernel offers a computationally efficient alternative to RPA-based approximations, enabling real-space dynamical methods to incorporate strong correlations without modifying long-range smoothed-charge electrostatics. Its analytic transparency bridges rigorous integral equation theory and practical dynamical kernels, additionally providing a physics-informed prior for emerging machine-learning models. Collectively, these results establish a mathematically rigorous testbed for advancing the modeling of strongly correlated soft matter systems.
6 February 2026







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Z
0
(
3
)
(the affine curve
Φ
3
(
X
,
Y
)
=
0
) together with its non-cuspidal singular points (nodes). Each node corresponds to a collision of two distinct points of
X
0
(
3
)
under the map
Γ
0
(
3
)
τ
↦
(
j
(
τ
)
,
j
(
3
τ
)
)
, i.e., to a self-intersection coming from two non-equivalent 3-isogenies
E
→
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′
. The plot was generated using Kainberger’s program [19].](https://mdpi-res.com/cdn-cgi/image/w=281,h=192/https://mdpi-res.com/axioms/axioms-15-00121/article_deploy/html/images/axioms-15-00121-g001-550.jpg)


