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First-principles molecular dynamics combined with the approach-to-equilibrium molecular dynamics methodology is employed to calculate the thermal conductivity of non-stoichiometric amorphous SiN. This is achieved by implementing thermal transients in five distinct models of different sizes along the direction of the heat transport. Such models have identical structural features and are representative of the same material, thereby allowing for a reliable analysis of thermal conductivity trends as a function of the relevant cell dimension. In line with the known physical law of heat propagation at short scale, the thermal conductivity increases in size with the direction of heat transport. The observed behavior is rationalized accounting for previous results on crystalline and amorphous materials, thus providing a unified description holding for a large class of materials and spanning a wide range of heat propagation lengths. [ABSTRACT FROM AUTHOR]

We present a hybrid similarity kernel that exemplifies the integration of short‐ and long‐range descriptors via the use of an average kernel approach. This technique allows for a direct measure of the similarity between amorphous configurations, and when combined with an active learning (AL) spectral clustering approach, it leads to a classification of the amorphous configurations into uncorrelated clusters. Subsequently, a minimum size database is built by considering a small fraction of configurations belonging to each cluster and a machine learning interatomic potential (MLIP), within the Gaussian approximation scheme, is fitted by relying on a Bayesian optimization of the potential hyperparameters. This step is embedded within an AL loop that allows to sequentially increase the size of the learning database whenever the MLIP fails to meet a predefined energy convergence threshold. As such, MLIP are fitted in an almost fully automatized fashion. This approach is tested on two diverse amorphous systems that were previously generated using first‐principles molecular dynamics. Accurate potentials with less than 2 meV/atom root mean square energy error compared to the reference data are obtained. This accuracy is achieved with only 175 configurations sampling the studied systems at various temperatures. The robustness of these potentials is then confirmed by producing models with several thousands of atoms featuring a good agreement with reference ab initio and experimental data. [ABSTRACT FROM AUTHOR]

Thermal properties are expected to be sensitive to the network topology, and however, no clearcut information is available on how the thermal conductivity of amorphous systems is affected by details of the atomic structure. To address this issue, we use as a target system a phase-change amorphous material (i.e., Ge2Sb2Te5) simulated by first-principles molecular dynamics combined with the approach-to-equilibrium molecular dynamics technique to access the thermal conductivity. Within the density-functional theory, we employed two models sharing the same exchange-correlation functional but differing in the pseudopotential (PP) implementation [namely, Trouiller–Martins (TM) and Goedecker, Teter, and Hutter (GTH)]. They are both compatible with experimental data, and however, the TM PP construction results in a Ge tetrahedral environment largely predominant over the octahedral one, although the proportion of tetrahedra is considerably smaller when the GTH PP is used. We show that the difference in the local structure between TM and GTH models impacts the vibrational density of states while the thermal conductivity does not feature any appreciable sensitivity to such details. This behavior is rationalized in terms of extended vibrational modes. [ABSTRACT FROM AUTHOR]

This article has to be intended both as a review and as an historical tribute to the ideas developed almost 40 years ago by S. Nosé, establishing the theoretical foundations of the implementation and use of thermostats in molecular dynamics (MDs). The original motivation of this work is enriched by an observation related to the connection between the Nosé seminal expression of temperature control and the extension proposed in 1985 by W. G. Hoover, known as the Nosé–Hoover thermostat. By carefully rederiving the equations of motion in both formalisms, it appears that all features of Nosé–Hoover framework (replacement of the Nosé variables by a single friction coefficient in the equations of motion) are already built in the Nosé approach. Therefore, one is able to work directly within the Nosé formalism with the addition of a single variable only, by greatly extending its general impact and simplicity and somewhat making redundant the Nosé–Hoover extension. Having been implicitly (and somewhat inadvertently) put to good use by a multitude of users over the past 40 years (in the context of classical and first‐principles MDs), this finding does not need any specific application to be assessed. [ABSTRACT FROM AUTHOR]

Sodium oxyhalide and hydroxyhalide antiperovskites are promising solid-state electrolytes (SSEs) because of their low melting point and rapid synthesis and, as such, they are becoming competitive with respect to other systems. While the structure and the mechanism underlying the ion dynamics are increasingly well understood in crystalline antiperovskites, their amorphous counterpart lacks precise structural characterization, hampering any conclusive insight into their properties. In this work, we resort to first-principles molecular dynamics within the Car–Parrinello scheme to assess the structure and ion dynamics of amorphous Na3−xOHxCl (with x = 0, 0.5, and 1) antiperovskites at a quantitative level. We obtain a detailed structural description of these amorphous systems, unveiling the mechanism inherent to the dynamics of Na ions, the role of H atoms, and the resulting ionic conductivity. Our results demonstrate that the structure of amorphous Na3OCl significantly differs from its crystal phase, showing very limited intermediate-range order and a short-range order mainly driven by four-fold Na atoms. Our results reveal that there is no evidence of phase separation in the amorphous Na3−xOHxCl, unlike the previous conjectured model of glassy Li3OCl. The amorphous structure of Na3OCl features remarkable Na ion dynamics and ionic conductivity, rivaling that of defective crystalline phases and highlighting its potential as a promising solid-state electrolyte. In hydroxylated models, the presence of hydroxyl OH− anions plays a crucial role in the mobility of Na ions. This is facilitated by the rapid rotation of O–H bonds and paddlewheel-type mechanisms, leading to enhanced ion mobility in the amorphous Na3−xOHxCl systems. This work provides unprecedented physical and chemical insight into the interplay between the structure, bonding, and ion transport in amorphous sodium-rich oxyhalide and hydroxyhalide antiperovskites, paving the way to their practical realization in next-generation SSEs. [ABSTRACT FROM AUTHOR]

Aset of structural properties of liquid GeSe2 are calculated by using first-principles molecular dynamics and including, for the first time, van derWaals dispersion forces. None of the numerous atomic-scale simulations performed in the past on this prototypical disordered network-forming material had ever accounted for dispersion forces in the expression of the total energy. For this purpose, we employed either the Grimme-D2 or the maximally localizedWannier function scheme. We assessed the impact of dispersion forces on properties such as partial structure factors, pair correlation functions, bond angle distribution, and number of corner vs edge sharing connections. The maximally localized Wannier function scheme is more reliable than the Grimme-D2 scheme in reproducing existing first-principles results. In particular, the Grimme-D2 scheme worsens the agreement with experiments in the case of the Ge-Ge pair correlation function. Our study shows that the impact of dispersion forces on disordered chalcogenides has to be considered with great care since it cannot be necessarily the same when adopting different recipes. [ABSTRACT FROM AUTHOR]

First-principles molecular dynamics simulations based on density functional theory are employed for a comparative study of structural and bonding properties of two stoichiometrically identical chalcogenide glasses, GeSe4 and GeS4. Two periodic cells of 120 and 480 atoms are adopted. Both glasses feature a coexistence of Ge-centered tetrahedra and Se(S) homopolar connections. Results obtained for N = 480 indicate substantial differences at the level of the Se(S) environment, since Ge-Se-Se connections are more frequent than the corresponding Ge-S-S ones. The presence of a more prominent first sharp diffraction peak in the total neutron structure factor of glassy GeS4 is rationalized in terms of a higher number of large size rings, accounting for extended Ge-Se correlations. Both the electronic density of states and appropriate electronic localization tools provide evidence of a higher ionic character of Ge-S bonds when compared to Ge-Se bonds. An interesting byproduct of these investigations is the occurrence of discernible size effects that affect structural motifs involving next nearest neighbor distances, when 120 or 480 atoms are used. [ABSTRACT FROM AUTHOR]