arXiv subjects

Alexander Schlaich

Publications and source records attributed to Alexander Schlaich.

9 recordsLinked to original sources

arXiv2026

Simultaneous Learning of Static and Dynamic Charges

Long-range interactions and electric response are essential for accurate modeling of condensed-phase systems, but capturing them efficiently remains a challenge for atomistic machine learning. Traditionally, these two phenomena can be represented by static charges, that participate in Coulomb interactions between atoms, and dynamic charges such as atomic polar tensors - aka Born effective charges - describing the response to an external electric field. We critically compare different approaches to learn both types of charges, taking bulk water and water clusters as paradigmatic examples: (1) Learning them independently; (2) Coupling static and dynamic charges based on their physical relationship with a single global coupling constant to account for dielectric screening; (3) Coupled learning with a local, environment-dependent screening factor. In the coupled case, correcting for dielectric screening is essential, yet the common assumption of homogeneous, isotropic screening breaks down in heterogeneous systems such as water clusters. A learned, environment-dependent screening restores high accuracy for the dynamical charges. However, the accuracy gain over independent dynamic predictions is negligible, while the computational cost increases compared to using separate models for static and dynamical charges. This suggests that, despite the formal connection between the two charge types, modeling them independently is the more practical choice for both condensed-phase and isolated cluster systems.

physics.chem-ph↗

arXiv2025

Static Dielectric Permittivity Profiles and Coarse-graining Approaches for Water in Graphene Slit Pores

The dielectric response of nano-confined fluids is crucial across technologies and biological systems, yet its calculation and interpretation from molecular simulations are often muddled by unclear boundary conditions. We re-derive the Green--Kubo relation for the spatially resolved linear dielectric response of fluids in planar confinement, explicitly accounting for boundary conditions and showing that equilibrium-derived profiles agree with those obtained from external fields. We identify common misconceptions in the literature and outline how microscopic dielectric behavior can be coarse-grained to connect with experimental observables. Simulations show that water retains a bulk-like dielectric response down to $\sim 1\,\mathrm{nm}$ confinement. The reduced \emph{effective} dielectric response that governs capacitance arises from the placement of the dielectric interface. Using effective-medium theory, we demonstrate that long-range reductions reported in experiments and theory are consistent with bulk-like behavior beyond about $1\,\mathrm{nm}$ from the surface. The effective response naturally maps onto an interfacial capacitance, and the dielectric properties of simulated water are robust across simulation setups and water models, reflecting universal polarization correlations.

cond-mat.soft↗

arXiv2025

Phase Diagram and Criticality of the Modified Primitive Electrolyte Model in Bulk and in Inert and Conducting Confinement

Ionic fluids under conductive confinement are central to technologies such as batteries, supercapacitors, and fuel cells. Their interfacial behavior governs energy storage and electrochemical processes. Despite their importance, the thermodynamics of even simple models -- such as the charged Lennard-Jones fluid -- remain underexplored in this regime. We present an extended Wang-Landau sampling approach to efficiently compute the density of states of charged mixtures with respect to the particle number. The method supports simulations in both bulk and confined geometries. Combined with the Constant Potential Method, it also enables to study effects due to confining electrodes. We employ this approach to study symmetric, binary mixtures of charged Lennard-Jones particles -- the modified Restricted Primitive Model -- in bulk, in inert confinement, and in conductive confinement at the potential of zero charge. Our results show that confinement shifts the vapor-liquid critical point to lower temperatures and higher densities compared to bulk, in line with the classical concept of capillary condensation. Importantly, conductive boundaries significantly lower the chemical potential of coexistence relative to inert confinement. These findings offer deeper insight into the phase behavior of ionic fluids in energy-relevant porous environments.

cond-mat.soft↗

arXiv2025

Intermittent molecular motion and first passage statistics for the NMR relaxation of confined water

The structure and dynamics of fluids confined in nanoporous media differ from those in bulk, which can be probed using NMR relaxation measurements. We here show, using atomistic molecular dynamics simulations of water in a slit nanopore, that the behavior of the NMR relaxation rate, R1, with varying surface interaction and confinement strength can be estimated from the exchange statistics of fluid molecules between the adsorbed surface layer and the bulk region, where molecules undergo intermittent dynamics. We employ first return passage time calculations to quantify the molecular exchange statistics, thereby linking microscopic parameters of the confined fluid-such as adsorption time, pore size, and diffusion coefficient-to the NMR relaxation rate. This approach allows to predict and interpret the molecular relaxation of fluids at interfaces using merely concepts of statistical mechanics and can be generalized to closed and open geometries.

cond-mat.soft↗

arXiv2024

Theory and Modeling of Transport for Simple Fluids in Nanoporous Materials: From Microscopic to Coarse-Grained Descriptions

We present the state-of-the-art theoretical modeling, molecular simulation, and coarse-graining strategies for the transport of gases and liquids in nanoporous materials (pore size 1-100 nm). Special emphasis is placed on the transport of small molecules in zeolites, active carbons, metal-organic frameworks, but also in nanoporous materials with larger pores such as ordered and disordered mesoporous oxides. We present different atomistic and mesoscopic methods as well as the theoretical formalisms. Attention is given to the investigation of different molecular transport coefficients - including the self, collective and transport diffusivities - but also to the determination of free energy barriers and their role in overall adsorption/separation process rates. We also introduce other available approaches such as hierarchical simulations and upscaling strategies. This review focuses on simple fluids in prototypical nanoporous materials. While the phenomena covered here capture the main physical mechanisms in such systems, complex molecules will exhibit additional specific features. For the sake of clarity and brevity, we also omit multicomponent systems (e.g. fluid mixtures, electrolytes, etc.) and electrokinetic effects arising when charged systems are considered (ionic species, charged surfaces, etc.), both of which add to the complexity.

cond-mat.soft↗

arXiv2024

Bridging Microscopic Dynamics and Hydraulic Permeability in Mechanically-Deformed Nanoporous Materials

In the field of nanoconfined fluids, there are striking examples of deformation/transport coupling in which mechanical solicitation of the confining host and dynamics of the confined fluid impact each other. While this intriguing behavior can be potentially used for practical applications (e.g. energy storage, phase separation, catalysis), the underlying mechanisms remain to be understood as they challenge existing frameworks. Here, using molecular simulations analyzed through concepts inherent to interfacial fluids, we investigate fluid flow in compliant nanoporous materials subjected to external mechanical stresses. We show that the pore mechanical properties significantly affect fluid flow as they lead to significant pore deformations and different density layering at the interface accounted for by invoking interfacial viscous effects. Despite such poromechanical effects, we show that the thermodynamic properties (i.e. adsorption) can be linked consistently to Darcy's law for the permeability by invoking a pore size definition based on the concept of Gibbs' dividing surface. In particular, regardless of the pore stiffness and applied external stress, all data can be rationalized by accounting for the fluid viscosity and slippage at the interface independent of a specific pore size definition. Using such a formalism, we establish that the intimate relation - derived using the linear response theory - between collective diffusivity and hydraulic permeability remains valid. This allows for linking consistently microscopic dynamics experiments and permeability experiments on fluid flow in compliant nanoporous materials.

physics.comp-ph↗

arXiv2022

Elucidating contact electrification mechanism of water

The open water surface is known to be charged. Yet, the magnitude of the charge and the physical mechanism of the charging remain unclear, causing heated debates across the scientific community. Here we directly measure the charge Q of microdrops ejected from hydrophilic and hydrophobic capillaries and show that the water surface can take both positive or negative charge values depending on pH and the capillary type. Our experiments, theory, and simulations provide evidence that a junction of two aqueous interfaces with a different ion adsorption energy (e.g., liquid-solid and liquid-air interfaces) develops a pH-dependent contact potential difference {\Delta}{\phi} up to 52 mV. The longitudinal charge transfer between the interfaces stimulated by {\Delta}{\phi} determines the charge of the open water surface. The suggested static electrification mechanism provides far-reaching insights into the origin of electrical potentials in biological and electrochemical energy systems.

cond-mat.soft↗

arXiv2021

Electronic screening using a virtual Thomas-Fermi fluid for predicting wetting and phase transitions of ionic liquids at metal surfaces

Of relevance to energy storage, electrochemistry and catalysis, ionic and dipolar liquids display unexpected behaviours-especially in confinement. Beyond adsorption, over-screening and crowding effects, experiments have highlighted novel phenomena, such as unconventional screening and the impact of the electronic nature-metallic versus insulating - of the confining surface. Such behaviours, which challenge existing frameworks, highlight the need for tools to fully embrace the properties of confined liquids. Here we introduce a novel approach that involves electronic screening while capturing molecular aspects of interfacial fluids. Although available strategies consider perfect metal or insulator surfaces, we build on the Thomas-Fermi formalism to develop an effective approach that deals with any imperfect metal between these asymptotes. Our approach describes electrostatic interactions within the metal through a 'virtual' Thomas-Fermi fluid of charged particles, whose Debye length sets the screening length $λ$. We show that this method captures the electrostatic interaction decay and electrochemical behaviour on varying $λ$. By applying this strategy to an ionic liquid, we unveil a wetting transition on switching from insulating to metallic conditions.

physics.chem-ph↗

arXiv2020

Molecular interpretation of the non-Newtonian viscoelastic behavior of liquid water at high frequencies

Using classical as well as ab-initio molecular dynamics simulations, we calculate the frequency-dependent shear viscosity of pure water and water-glycerol mixtures. In agreement with recent experiments, we find deviations from Newtonian-fluid behavior in the THz regime. Based on an extension of the Maxwell model, we introduce a viscoelastic model to describe the observed viscosity spectrum of pure water. We find four relaxation modes in the spectrum which we attribute to i) hydrogen-bond network topology changes, ii) hydrogen-bond stretch vibrations of water pairs, iii) collective vibrations of water molecule triplets, and iv) librational excitations of individual water molecules. Our model quantitatively describes the viscoelastic response of liquid water on short timescales, where the hydrodynamic description via a Newtonian-fluid model breaks down.

physics.flu-dyn↗