arXiv subjects

Samuel Varner

Publications and source records attributed to Samuel Varner.

5 recordsLinked to original sources

arXiv2026

Stockmayer Fluid with a Shifted Dipole. III. Ion Solvation

Though ion solvation is well understood for the standard Stockmayer fluid, the effect of displacing the point dipole of a Stockmayer particle, a simple geometric modification, on ion-solvent structuring and solvation thermodynamics remains less understood. We use molecular dynamics simulations to describe how a dipole shift affects radial and orientational solvent structure around centered ions, how strongly the solvent distinguishes cations and anions, and how the solvent mediates ion-ion pairing. We show that, with a centered dipole, cations and anions have equal preferences for the local solvent environment. Shifting the dipole breaks the fore-aft symmetry of the local electrostatic field, leading to a stronger preference for a Stockmayer particle's dipole to orient towards an anion and a weaker preference to orient away from a cation. These structural asymmetries propagate to the solvation free energies: increasing dipole displacement makes anion solvation more favorable, while cation solvation becomes less favorable overall across dipole strengths. The same asymmetry also manifests in the effective ion-ion interaction-increasing the dipole shift reorganizes the local solvation environment, enhancing solvent shielding of short-range cation-anion contact while weakening screening at longer range. As a result, solvent-separated configurations become comparatively more favorable, whereas increasing dipole strength produces the opposite trend. Such effects cannot be explained by bulk dielectric screening alone, indicating that molecular charge geometry plays an important role in both single-ion solvation and solvent-mediated ion-ion interactions. This work demonstrates that dipole location, not only magnitude, provides a geometric control on ion-specific solvation and ion pairing in general polar fluids.

cond-mat.stat-mech↗

arXiv2026

NEMORA: Neural Equivariant Multipole Operators for Long-Range Atomistic Learning

Equivariant graph neural networks have emerged as foundational architectures for machine-learned interatomic potentials, approaching quantum-chemical accuracy at a fraction of the computational cost. These models describe local atomic environments accurately, but finite spatial cutoffs truncate long-range information flow, and stacking message-passing layers can lead to over-smoothing and over-squashing. Existing long-range extensions either prescribe a fixed analytical propagation kernel, restrict long-range communication to scalars or degree-preserving channels, are only approximately equivariant, or incur super-linear computational cost. Combining learnable long-range equivariant transport with multiscale many-body expressivity and efficient scaling for larger systems remains a central challenge. We introduce Neural Equivariant Multipole Operators (NEMORA), a neural equivariant extension of the Fast Multipole Method (FMM) for learning long-range tensorial representations. NEMORA generalizes the FMM's analytical multipole expansion and translation operators to learned equivariant counterparts on an adaptive spatial hierarchy. Its operators couple angular degrees and form many-body interactions across length scales, retaining the FMM's hierarchical organization and analytical radial factors as physical inductive biases while learning data-dependent long-range couplings. NEMORA evaluates in linear time and memory complexity, allowing it to treat larger systems than other long-range methods reaching hundreds of thousands of atoms, and it augments both symmetry-constrained and unconstrained short-range backbones. On non-local benchmarks, it reduces force and energy errors relative to the short-range backbones by over an order of magnitude and up to three orders of magnitude, respectively, which is better than or competitive with existing long-range extensions in accuracy.

cs.LG↗

arXiv2026

Stockmayer Fluid with a Shifted Dipole: Bulk Behavior

Shifting the point dipole from the center of a Stockmayer particle is a simple geometric modification that has been explored previously, yet its implications for liquid structure, dielectric response, and phase behavior remain incompletely understood. Here, we combine molecular dynamics simulations with analytical theory to provide a unified physical interpretation of how dipole displacement reshapes microscopic correlations and propagates to macroscopic thermodynamic properties. We show that dipole shifting breaks the fore-aft symmetry of the local electrostatic field, producing only modest changes in radial packing but strong alterations in angular structure within the first solvation shell. Enhanced alignment near the dipole head is accompanied by frustrated orientational correlations near the tail, leading to broader angular distributions and a shift away from axial configurations at strong coupling. These structural asymmetries weaken cooperative ordering and result in a systematic reduction of the dielectric constant, despite locally stronger interactions. For large shifts, the dielectric response approaches the Debye limit, indicating effective suppression of dipole-dipole correlations. The same geometric frustration governs vapor-liquid equilibria: while increasing dipole strength raises the critical temperature, even modest shifts disrupt the highly polarized liquid states that emerge at strong coupling and can suppress ferroelectric-like ordering. Predictions from a reparameterized COFFEE theory capture these trends within its domain of validity, highlighting the direct connection between local orientational structure and macroscopic observables. Overall, this work demonstrates that dipole location, not only magnitude, provides a powerful control parameter in dipolar fluids and offers a clear framework for understanding geometric frustration in electrostatic liquids.

cond-mat.soft↗

arXiv2025

Mechanisms of Chain Exchange in Diblock Copolymer Micelles

We investigate the mechanism of chain exchange in diblock copolymer micelles using two distinct yet complementary simulation techniques. First, enhanced sampling method is combined with coarse-grained molecular dynamics to compute a two-dimensional free energy surface for the chain expulsion process in the strong segregation regime. To facilitate chain expulsion, a distance-based collective variable is biased, and the end-to-end distance of the core block is also biased to ensure sufficient sampling of chain conformations. The free energy surface reveals a bimodal distribution of chain conformations along the effective reaction coordinate. The minimum free energy pathway qualitatively aligns with the Halperin-Alexander budding-like mechanism. The free energy barrier along this pathway is calculated for core block lengths ranging from $N_\textrm{core}=4$-$100$, and the barrier is shown to scale as $βΔF_\textrm{barr} \sim N_\textrm{core}^{2/3}$, consistent with the Halperin-Alexander prediction for a globular transition state. Notably, the free energy surface also reveals a nearly degenerate alternative pathway in which the chain escapes by extending out bead-by-bead, in agreement with previous simulations. We also study the case of a dense copolymer melt, where the core-block shrinks but does not collapse into a dry compact globule in the opposite phase. To examine the kinetic pathway, a simplified model is introduced in which a single chain escapes from a planar interface within a mean-field background. Forward flux sampling calculations yield a linear scaling of the barrier, $βΔF_\textrm{barr} \sim N_\textrm{core}$, in agreement with experimental observations and prior simulations. Moreover, analysis of successful escape trajectories highlights an extended chain conformation at the transition state.

cond-mat.soft↗

arXiv2025

Stockmayer Fluid with a Shifted Dipole: Interfacial Behavior

We investigate the properties of the liquid-vapor interface in the shifted Stockmayer fluid using molecular dynamics simulations in the canonical ensemble. We study the role of the dipole moment strength and the degree of asymmmetry on equilibrium interfacial characteristics, including density profiles, polar order, nematic order, interfacial polarization, electric field, and electrostatic potential. In addition, we compute angular distribution functions across the interface to gain insight into how the dipole shift affects the molecular orientation. We find that the shift significantly effects angular distribution functions by altering the polar order while leaving the nematic order relatively unaffected, in comparison to the reference symmetric Stockmayer fluid. We find that these results are consistently explained using an image-dipole construction that has been previously applied to symmetric Stockmayer fluids but has never been extended to the shifted model. We find remarkable agreement between the simple theory and the simulations in the qualitative shape of the distribution functions for both the liquid and vapor phases in proximity to the interface. Unexpectedly, the spontaneous polarization at the interface, and therefore the generated electric field, changes sign as the dipole moment strength increases. This also leads to an inversion of the sign of the potential difference across the interface.

cond-mat.soft↗