Condensed Matter Physics contains original and review articles in the field of statistical mechanics and thermodynamics of equilibrium and nonequilibrium processes, relativistic mechanics of interacting particle systems.The main attention is paid to physics of solid, liquid and amorphous systems, phase equilibria and phase transitions, thermal, structural, electric, magnetic and optical properties of condensed matter. Condensed Matter Physics is a diamond open-access, international peer-reviewed scientific journal published quarterly. ISSN 1607-324X (print), 2224-9079 (online) Key title: Condensed Matter Physics Abbreviated key title: Condens. Matter Phys.
Yukhnovskii Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine
Enhanced approach to calculation of cluster integrals for lattice models of matter
The study is devoted to enhancing the existing techniques of calculating Mayer’s expansion cluster integrals for lattice models of matter. Two important optimizations are proposed: simplifying the calculation of the integrand at each integration point and reducing the number of such integration points due to eliminating physically identical configurations. Based on those optimizations, new data on high-order cluster integrals are obtained for a number of 2D and 3D lattice models.
Copyright (c) 2026 M. V. Ushcats, S. Yu. Ushcats https://creativecommons.org/licenses/by/4.0
On the electrical double layer capacitance of the restricted primitive model: a link between the mesoscopic theory and the associative mean spherical approximation
The results for the electrical double layer capacitance and the density of “free ions” obtained from the mesoscopic theory are compared with the corresponding results of the associative mean spherical approximation. While the first theory takes into account the fluctuations of the charge density, the second theory assumes that the free ions and ion pairs are in chemical equilibrium according to the mass action law. Our results demonstrate a fairly good agreement between the two theories at high densities and low temperatures.
associative mean spherical approximation
Copyright (c) 2026 O. Patsahan https://creativecommons.org/licenses/by/4.0
Halide substitution effects on the photovoltaic properties of Ca3PX3 (X = F, Cl, Br, I) perovskites: advancing solar cell efficiency
Herein, the fundamental physical characteristics like structural, electronic, optical parameters of the Ca3PX3 (X = F, Cl, Br, I) materials have been investigated for their potential optoelectronic applications, particularly for solar cells and related devices. To the crystallographic investigations, Ca3PI3 has the most stable configuration among all investigated materials. From the band structure analyses of these materials indicate that all materials have a direct bandgap in the range of 2.0 eV to 3.788 eV, which makes them ideal for light absorption. For the photovoltaic applications, we have analysed first-principles spectroscopic screening limited maximum efficiency (SLME) which confirms that the Ca3PI3 material exhibits the highest solar cell efficiency 29.6% and Ca3PF3 and shows lower efficiency for solar cell suitability 0.6%. Thus, these results demonstrate the real potential and abilities of halide substitution to tune the materials for particular optoelectronic devices.
Copyright (c) 2026 P. Dhariwal, D. Prakash, K. D. Verma, A. Kumari, P. K. Kamlesh, A. S. Verma https://creativecommons.org/licenses/by/4.0
First-principles study of the impact of as doping on the structural and electronic properties of MoS2 monolayer
This study is aimed at exploring the structural and electronic properties of doped MoS2 monolayers, including Mo and S vacancies and As doped systems, employing DFT calculations. The electronic properties were analyzed to understand how these modifications affect the behavior of the material. Introduction of defects generates new defect states in the midgap. In the S-vacancy (VS), Mo- vacancy (VMo), As-Mo (As substituting Mo), and As-S (As substituting S) doped systems, the downward shift of the Fermi level to the valence band indicates a p- type behavior. In the As interstitial system the Fermi level shifts to the conduction band, suggesting an n-type semiconductor. The results highlight that doping MoS2 with As, particularly at the Mo site, can be used in photocatalysis and high-efficiency photovoltaics. Additionally, the As interstitial system demonstrates an enhanced performance in field-effect transistors (FETs).
Copyright (c) 2026 A. Daouadi, M. L. Benkhedir https://creativecommons.org/licenses/by/4.0
Spectral and thermodynamic properties of supersymmetric quantum systems with self-adjoint deformed momentum
We establish a rigorous framework for quantum systems with geometric deformations by constructing a strictly self-adjoint deformed momentum operator through the generalized extended momentum operator (GEMO) formalism. Unlike previous approaches relying on boundary-condition hermiticity, our method ensures intrinsic self-adjointness for both linear (μ(x)=αx) and quadratic (μ(x)=αx 2) deformations within a unified non-Hermitian supersymmetric factorization scheme. This yields exact analytical spectra while revealing hidden su (1, 1) symmetry structures. Crucially, we provide the first complete thermodynamic characterization of such systems by analytically evaluating the partition function via the Euler-Maclaurin approximation. Geometric deformation fundamentally reshapes the density of states ρ(E), producing distinct thermal signatures: a divergent heat capacity peak for linear deformation due to state accumulation near a maximal energy, and a saturation C/kB → 0.6 (below the Dulong-Petit limit) for quadratic deformation. These results establish geometric deformation as a tunable parameter for engineering quantum thermodynamic responses in curved nanostructures.
Copyright (c) 2026 J. A. Oke, F. A. Dossa https://creativecommons.org/licenses/by/4.0
On the statistical theory of strong electrolytes and high-temperature plasmas: new applications of the work of Yukhnovskii and Kelbg
Remembering here the work of two pioneers of the statistical physics of Coulomb systems, Günter Kelbg, and Ihor Yukhnovskii, we analyze their methods and give some new applications to ionic solutions and quantum plasmas. In particular, we develop applications of the theory to strong electrolytes and to thermal high-temperature plasmas at T > 105 K using the exponential interaction model. We show the strong structural similarity of these two classes of Coulomb systems, which physics is determined mostly by contributions proportional to e4 and e6. We predict at higher densities a structural transition to oscillating correlations. The thermodynamic functions show a smooth transition from a quadratic root increase to a slower increase like ni1/4 which observes the Onsager bound. Effects of asymmetries in charges and masses are studied with applications to ionic systems with multiple charges and to high-temperature plasmas, in particular, to plasmas with He2+-ions.
Сollection of the articles dedicated to the 100th anniversary of Prof. Ihor Yukhnovskii
Copyright (c) 2026 W. Ebeling, M. Holovko https://creativecommons.org/licenses/by/4.0
Effect of weak positional disorder on the miniband structure of spherical quantum dot chains
A theoretical framework is developed for the electron miniband structure in one-dimensional chains of spherical quantum dots subjected to weak positional disorder. Within the tight-binding approximation and effectivemedium approach, stochastic fluctuations of the inter-dot spacing are mapped onto the renormalization of key Hamiltonian parameters: the hopping integral B, overlap integral Q, and on-site energy shift M. Analytical expressions for these parameters are derived by performing an ensemble average over a narrow Gaussian distribution of positional deviations (σ ≪ a). The resulting generalized dispersion relation reveals that weak positional disorder broadens the minibands. For typical fabrication fluctuations σ = 0.1a, the miniband width increases by 8-12% , depending on the mean inter-dot distance a. This sensitivity to disorder decreases rapidly with increasing lattice period due to the exponential decay of electron wave functions. In this weak-disorder regime, the Anderson localization lengh significantly exceeds the lattice constant, meaning states remain delocalized.
Copyright (c) 2026 R. Ya. Leshko https://creativecommons.org/licenses/by/4.0
Thermodynamic stability and structural transitions in virus–host networks
Understanding virus–host interactions is crucial for predicting the stability of networks under various perturbations. In this study, we present an analysis of virus-related networks for several organisms (Homo sapiens, Mus musculus, Gallus gallus), encompassing directed and weighted connections. We compute a range of network parameters, including topological characteristics and thermodynamic quantities derived from adjacency spectra, to gain insights into the structural robustness and dynamic behavior of the networks. To assess stability, we model two distinct node removal scenarios: targeted elimination of the most influential nodes and random removal. Our findings reveal transition-like behavior in spectral thermodynamic functions and characteristic changes in structural measures, contributing to evaluating the potential of a thermodynamic framework for studying virus–host networks and advancing a deeper understanding of their dynamics.
Copyright (c) 2026 A. Rovenchak, M. Husiev https://creativecommons.org/licenses/by/4.0
An elastic model of confined hydrogel particles with competing entropic and energetic networks
This work presents an elastic model to study the interplay between entropic and energetic networks in confined hydrogel particles. We consider a quasi-two-dimensional system composed of spherical hydrogel beads confined in a circular container, where particle growth occurs through hydration. Based on experimental observations, an elastic potential is introduced to model interactions between particles and between particles and the confining wall. Computational simulations based on energy minimization identify the lowest-energy configurations adopted during growth. Analysis of the resulting energy landscapes reveals emergent self-organization, adaptability, and cooperativity arising from the competition between entropic and energetic networks.
Special issue dedicated to the 75th anniversary of Prof. Stefan Sokołowski
Copyright (c) 2026 A. Trokhymchuk, A. Huerta, L. A. Perez https://creativecommons.org/licenses/by/4.0
Field-tunable quadruple-Q states driven by momentum-space frustration
Multiple-Q magnetism in itinerant electron systems enables complex spin crystals and noncoplanar textures even in centrosymmetric settings. We study a minimal momentum-space spin model on a square lattice with four symmetry-related ordering wave vectors, including bilinear and biquadratic interactions under an out-ofplane magnetic field. Using simulated annealing, we obtain the field-dependent phase diagram and identify successive transitions among single-Q, double-Q, and multiple inequivalent quadruple-Q states. The quadruple-Q manifold exhibits rich internal structures: the states sharing the same wave vectors differ in phase locking, amplitude distribution, and noncoplanarity, leading to distinct real-space textures and scalar spin chirality patterns. Our results demonstrate that momentum-space frustration and biquadratic coupling provide an efficient route to stabilizing diverse quadruple-Q spin crystals, offering a general framework for higher-order spin textures in centrosymmetric itinerant magnes.
Copyright (c) 2026 S. Hayami https://creativecommons.org/licenses/by/4.0
Monte Carlo simulation of selective adsorption in a binary hard-disk mixture on patterned adhesive surfaces
Selective adsorption in a two-dimensional model of a binary hard-disk mixture on patterned adhesive surfaces is studied using grand canonical Monte Carlo simulations. The two species have equal diameters and equal bulk chemical potentials, but different attraction strengths to adhesive domains. Thus, affinity-driven selectivity is separated from particle-size asymmetry and unequal chemical potentials. The surface pattern is defined by domain size, domain surface coverage, and ordered or disordered arrangement of circular domains. The results show that selectivity strongly depends on surface geometry, especially at low and intermediate chemical potentials. Domains comparable to the particle size enhance selectivity by forming adsorption regions with large particle–domain overlap, whereas larger domains can provide high selectivity at low chemical potentials. For small domains, further reduction in size can also increase selectivity as the system approaches a uniform attractive surface with corresponding effective affinity parameters of the species.
Copyright (c) 2026 N. Kukarkin, T. Patsahan https://creativecommons.org/licenses/by/4.0
Statistical Mechanics of Coulomb Systems by Angel Alastuey and Philippe A. Martin
Copyright (c) 2026 W. Ebeling https://creativecommons.org/licenses/by/4.0
The influence of dimensional crossover on phase transitions and critical phenomena in condensed systems
This article is aimed at studying the effects of the dimensional crossover (DC) on physical properties of condensed systems near phase transition and critical points. Here we consider the following problems: (1) the theoretical provisions that allow to study the effect of spatial confinement on DC near phase transition and critical points; (2) the study of DC in condensed systems with the Ginzburg number Gi < 1, where fluctuation effects are described in different ways at the fluctuation, regular and intermediate (crossover) regions; (3) two types of DC were investigated: (a) a decrease in the linear dimensions L to the values of the correlation length of the order parameter fluctuations leads to the conversion of the dependence on thermodynamic variable into a dependence on linear sizes of 3D systems, as well as (b) a further decrease in linear sizes L the 3D–2D or 3D1D DC happens depending on slitlike or cylindrical geometry, which is determined by the value of the lower crossover dimensionality dLCD; (4) it is proposed to extend the known equalities for critical exponents by using the Mandelbrot formula for fractal dimension Df as a critical exponent; (5) the influence of 3D–2D DC on the characteristics of the fine structure of the molecular light scattering (MLS) spectrum is studied.
Copyright (c) 2025 O. V. Chalyi, E. V. Zaitseva https://creativecommons.org/licenses/by/4.0
Modelling SARS-CoV-2 epidemics via compartmental and cellular automaton SEIRS model with temporal immunity and vaccination
We consider the SEIRS epidemiology model with such features of the COVID-19 outbreak as: abundance of unidentified infected individuals, limited time of immunity and a possibility of vaccination. The control of the pandemic dynamics is possible by restricting the transmission rate, increasing identification and isolation rate of infected individuals, and via vaccination. For the compartmental version of this model, we found stable diseasefree and endemic stationary states. The basic reproductive number is analysed with respect to balancing quarantine and vaccination measures. The positions and heights of the first peak of outbreak are obtained numerically and fitted to simple in usage algebraic forms. Lattice-based realization of this model is studied by means of the asynchronous cellular automaton algorithm. This permitted to study the effect of social distancing by varying the neighbourhood size of the model. The attempt is made to match the quarantine and vaccination effects.
Copyright (c) 2026 J. Ilnytskyi, T. Patsahan https://creativecommons.org/licenses/by/4.0
How does ethane wet different substrates?
Computer simulations are employed to investigate the adsorption mechanisms of ethane on both homogeneous and inhomogeneous substrates. For homogeneous surfaces, the full range of surface phase transitions—from incomplete to complete wetting — can be accessed by tuning the strength of the surface potential. The resulting layering transition temperatures show excellent agreement with experimental measurements of ethane on graphite. By contrast, although all inhomogeneous substrates exhibit a prewetting transition, the adsorption mechanisms are strongly influenced by the stripe width.
Copyright (c) 2026 Ł. Baran, D. Tarasewicz, W. Rżysko https://creativecommons.org/licenses/by/4.0
Pressure effects in the properties of simple monohydric alcohols. Lessons from molecular dynamics simulations of united atom type UAM-EW model
We explore the pressure dependence of a set of properties of simple monohydric alcohols, namely of methanol, ethanol and 1-propanol, by using isobaric-isothermal molecular dynamics computer simulations. A recently proposed united atom, non-polarizable force field for each of alcohols [V. García-Melgarejo et al., J. Mol. Liq., 323, 114576 (2021)] is applied. Accuracy of the force field is evaluated by comparing the simulation results and available experimental data from the literature. Specifically, the density of alcohols upon increasing pressure, the isothermal compressibility, the static dielectric constant and self-diffusion coefficient are investigated starting from 1 bar up to 3 kbar. Evolution of the microscopic structure under pressure is discussed in terms of the pair distribution functions and some coordination numbers. Conclusions of the present modelling and necessary developments to consider in future work are commented on.
Copyright (c) 2026 M. Aguilar, L. Pusztai, O. Pizio https://creativecommons.org/licenses/by/4.0
Surfactant solutions confined in homogeneous and Janus-like slits
We study the behavior of aqueous surfactant solutions in the bulk phase and in slit-like pores by molecular dynamics. Adsorption and self-assembly of nonionic surfactants C7E3 that mimic alkyl poly(ethylene oxide) molecules are investigated. We consider pores with the same walls and Janus-like slits. The individual walls are inert, hydrophilic, or hydrophobic. We focus on the morphology of the surfactant solution confined in different slits. The influence of a pore type and its width is discussed. The aggregative adsorption of surfactants was found. Our simulations show that in slits surfactants assemble into structures that do not occur in the bulk phases.
Copyright (c) 2026 T. Staszewski, M. Borówko https://creativecommons.org/licenses/by/4.0
Molecular dynamics study of perchloric acid using the extended Madrid-2019 force field
Perchloric acid (HClO4) is widely used to prepare perchlorate salts with applications in propellants, industry, environmental chemistry, and biology. In this work, we used the intermolecular parameters from the extended Madrid-2019 force field for the perchlorate anion (ClO4-) and the oxonium cation (H3O+) together with TIP4P/2005 water to model perchloric acid solutions. The force field uses scaled charges of ±0.85e for monovalent ions and has been widely applied for aqueous ionic systems. We used the model to predict thermodynamic properties [densities and temperatures of maximum in density (TMD)], structural features (ion-water correlations: ion-hydrogen and ion-oxygen), and transport properties (self-diffusion coefficients and viscosity) of perchloric acid solutions at several concentrations. Experimental densities are predicted in excellent agreement up to 10 m. We also performed molecular simulations over a wide range of temperatures in order to determine the TMD of perchloric acid at different mol lities. Predicted viscosities at 298.15 K and 1 bar are in good agreement with experimental data for concentrations below 4 m. Results are discussed in terms of model strengths and limitations.
Copyright (c) 2026 M. Cruz-Sánchez, S. Blazquez, C. Vega, V. M. Trejos https://creativecommons.org/licenses/by/4.0
Symmetric mixtures in slit-like pores with selective walls
Symmetric mixtures characterized by high negative geometric and energetic non-additivity do not exhibit phase separation in the bulk. However, the phase separation occurs when such mixtures are confined in slit pores with selective walls. It is demonstrated that the wall selectivity affects the pore filling. When the difference of the interaction energies between the mixture components and pore walls is lower than a certain threshold value, condensation occurs between a dilute phase and the mixed liquid. When this difference exceeds the threshold value, the pore filling may occur in two steps. The first is the condensation of a dilute phase into the demixed liquid, and the second step leads to the formation of the mixed liquid. We have elucidated the changes in the phase behavior caused by non-additivity of symmetric mixtures, and by the difference in the interaction energies of the components with pore walls.
Copyright (c) 2026 A. Patrykiejew https://creativecommons.org/licenses/by/4.0
The application of Kirkwood-Buff theory to study hydration properties of α-amino acids
Protein conformational stability and function depend on non-covalent interactions that are strongly influenced by the surrounding environment. To explore protein properties, amino acids are often utilized as model systems. In this study, we determined the densities of seven α-amino acids in aqueous solutions between 278.15 K and 308.15 K and calculated the apparent molar volumes. Linear extrapolation yielded standard molar volumes, which were analyzed to characterize amino-acid hydration. The contributions of side chains to the standard molar volume were determined relative to glycine. The standard molar volume increased with temperature, indicating reduced electrostriction of water around the amino acids, consistent with lower hydration numbers at higher temperatures. We employed the Ornstein-Zernike integral equation with hypernetted-chain closure and a coarse-grained Lennard-Jones bead model to calculate pair correlation functions and Kirkwood-Buff integrals, from which standard molar volumes were obtain. The model reproduced the experimental standard molar volumes very well.
Copyright (c) 2026 Z. Štefanič, B. Hribar-Lee https://creativecommons.org/licenses/by/4.0
Multiscale theory, modelling, and simulation of hemicellulose and lignin in solution
This review examines multiscale modelling approaches for cellulose nanocrystals (CNCs) and lignocellulosic plant cell walls, with a focus on hemicellulose and lignin interactions in aqueous environments. The threedimensional reference interaction site model with the Kovalenko–Hirata closure (3D-RISM-KH) is highlighted as a powerful molecular solvation theory applied in nanochemistry and biomolecular simulations. The method has been successfully employed to investigate hemicellulose hydrogels, the influence of hemicellulose composition on nanoscale forces in primary cell walls, and lignin-lignin and lignin-hemicellulose interactions. Findings indicate that these interactions are predominantly hydrophobic and entropy-driven, arising from water exclusion effects. Insights gained through this modeling framework deepen the understanding of molecular-scale forces in plant cell walls and inform strategies for biomass valorization, including genetic engineering and pretreatment technologies aimed at enhancing cellulose extraction and utilization.
Copyright (c) 2026 A. Kovalenko https://creativecommons.org/licenses/by/4.0
Vortex formation in the Vicsek model with internal chirality of self-propelling objects
Effect of internal chirality on collective motion of a large number of active objects is studied by simulations of appropriately modified Vicsek model. We add a fixed angle to the noise and consider small ratios, p, between this angle and the maximal deviation from the average local direction of motion. When the above ratio is p = 1/120, the traveling bands observed with the symmetrical noise are destroyed, and small bands moving in different directions appear. Circular rotating flocks of objects with the same orientation are formed for p = 1/7.5. Stable vortexes in the stationary state were found from p = 1/60 to p = 1/20. Velocity autocorrelation function shows equilibrium between the inflow and the outflow to and from the vortex. Long-time evolution is considerably influenced by a temporary trapping of the objects in the vortex. The ballistic behavior for the symmetrical noise changes to the diffusive behavior for the chirality leading to the onset of vortexes.
Copyright (c) 2026 A. Ciach, W. Gozdz https://creativecommons.org/licenses/by/4.0
II. Temperature trends in the properties of simple monohydric alcohols. Molecular dynamics simulations of united atom UAMI-EW model
We explore the dependence of a wide set of properties of monohydric alcohols on temperature by using the isobaric-isothermal molecular dynamics computer simulations. Namely, methanol (MeOH), ethanol (EtOH) and 1-propanol (PrOH) alcohols are studied. The recently proposed united atom, non-polarizable force field for each of alcohols [V. García-Melgarejo et al., J. Mol. Liq., 2021, 323, 114576] is applied for this purpose. Accuracy of the force field is discussed comparing predictions from simulations and experimental data for density, dielectric constant, surface tension, and self-diffusion coefficient. Supplementary insights concerning applicability of the model are obtained by exploration of the composition dependence of various properties for MeOH–PrOH mixtures. Peculiarities of mixing of species in this system are elucidated in terms of density, excess mixing volume and excess mixing enthalpy. Static dielectric constant of the mixture and the corresponding excess are obtained. Perspectives of modelling are commented finally.
Copyright (c) 2026 M. Aguilar, E. Núñez-Rojas, O. Pizio https://creativecommons.org/licenses/by/4.0
Thermodynamics of hard-sphere fluids in polydisperse random porous media: Extended scaled particle theory
Accurate descriptions of reference systems are a central task in liquid-state theories for the study of more complex systems. Using scaled particle theory (SPT), we derive a fully analytical description of the thermodynamic properties of a hard-sphere (HS) fluid confined in size-polydisperse HS random porous media, extending the existing approaches to higher matrix packing fractions. We calculate chemical potentials for a wide range of porousmatrix parameters, including the matrix packing fraction, degree of polydispersity, and particle-size distributions. Within the proposed framework, our results show excellent agreement with available Monte Carlo simulations and previous integral-equation theories over a broad range of matrix packing fractions, 0.1 ≤ η0 ≤ 0.3, and degrees of polydispersity.
Copyright (c) 2026 T. Hvozd, M. Hvozd, M. Holovko https://creativecommons.org/licenses/by/4.0
Behaviour of the model antibody fluid constrained by rigid spherical obstacles: effects of the obstacle-antibody binding
We study a simplified model of monoclonal antibodies confined in a patchy random porous medium. Antibodies are represented as Y-shaped particles composed of seven tangential hard spheres with attractive patches on the terminal beads, while the matrix consists of randomly distributed hard-sphere obstacles bearing adhesive sites. The model captures antibody behavior in crowded biological environments with strong short-range antibodymatrix attractions. The theoretical approach combines Wertheim’s multidensity thermodynamic perturbation theory, the Flory-Stockmayer theory of polymerization, and scaled particle theory for fluids in porous media. We analyze thermodynamic properties, percolation thresholds, and phase behavior, and compare the selected results with new computer simulations. The interplay between antibody-antibody and antibody-matrix interactions produces a complex phase behavior, including re-entrant phase separation with a closed-loop coexistence region at higher temperatures and conventional liqui-gas separation at lower temperatures.
Copyright (c) 2026 Yu. Kalyuzhnyi, T. Patsahan https://creativecommons.org/licenses/by/4.0
Advances in the microscopic theory of inhomogeneous fluids: A tribute to Stefan Sokołowski on his 75th Birthday
This special issue is a collection of papers written by friends and colleagues of Prof. Stefan Sokołowski in memory of 75 years from his birth. We remember and miss him. This foreward contains a brief description of scientific contributions by Stefan principally focused in the construction of microscopic theory of inhomogeneous fluids. It refers mostly to our happy times while working together alog some of the topics mentioned below.We apologize that several issues of scientific interest have not been discussed in detail. Nevertheless, we hope that this text reflects motto of Stefan’s personality and besides will attract the attention of younger readers to a set of problems unsolved so far.
Copyright (c) 2026 J. Ilnytskyi, T. Patsahan, O. Pizio https://creativecommons.org/licenses/by/4.0
Copyright (c) 2026 https://creativecommons.org/licenses/by/4.0
The 70th Birthday of DSc Oksana Patsahan
On the 70th anniversary of Professor Tony Haymet