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Traditional classifications of crystalline phases focus on nuclear degrees of freedom. Through examination of both electronic and nuclear structure, we introduce the concept of an electronic plastic crystal. Such a material is classified by crystalline nuclear structure, while localized electronic degrees of freedom - here lone pairs - exhibit orientational motion at finite temperatures. This orientational motion is an emergent phenomenon arising from the coupling between electronic structure and polarization fluctuations generated by collective motions, such as phonons. Using ab initio molecular dynamics simulations, we predict the existence of electronic plastic crystal motion in halogen crystals and halide perovskites, and suggest that such motion may be found in a broad range of solids with lone pair electrons. Such fluctuations in the charge density should be observable, in principle via synchrotron scattering.
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Many atomic liquids can form transient covalent bonds reminiscent of those in the corresponding solid states. These directional interactions dictate many important properties of the liquid state, necessitating a quantitative, atomic-scale understanding of bonding in these complex systems. A prototypical example is liquid silicon, wherein transient covalent bonds give rise to local tetrahedral order and consequent non-trivial effects on liquid state thermodynamics and dynamics. To further understand covalent bonding in liquid silicon, and similar liquids, we present an ab initio simulation-based approach for quantifying the structure and dynamics of covalent bonds in condensed phases. Through the examination of structural correlations among silicon nuclei and maximally localized Wannier function centers, we develop a geometric criterion for covalent bonds in liquid Si. We use this to monitor the dynamics of transient covalent bonding in the liquid state and estimate a covalent bond lifetime. We compare covalent bond dynamics to other processes in liquid Si and similar liquids and suggest experiments to measure the covalent bond lifetime.
The dynamics of desorption from a submonolayer of adsorbed atoms or ions are significantly influenced by the absence or presence of lateral diffusion of the adsorbed particles. When diffusion is present, the adsorbate configuration is simultaneously changed by two distinct processes, proceeding in parallel: adsorption/desorption, which changes the total adsorbate coverage, and lateral diffusion, which is coverage conserving. Inspired by experimental results, we here study the effects of these competing processes by kinetic Monte Carlo simulations of a simple lattice-gas model. In order to untangle the various effects, we perform large-scale simulations, in which we monitor coverage, correlation length, and cluster-size distributions, as well as the behavior of representative individual clusters, during desorption. For each initial adsorbate configuration, we perform multiple, independent simulations, without and with diffusion, respectively. We find that, compared to desorption without diffusion, the coverage-conserving diffusion process produces two competing effects: a retardation of the desorption rate, which is associated with a coarsening of the adsorbate configuration, and an acceleration due to desorption of monomers evaporated from the cluster perimeters. The balance between these two effects is governed by the structure of the adsorbate layer at the beginning of the desorption process. Deceleration and coarsening are predominant for configurations dominated by monomers and small clusters, while acceleration is predominant for configurations dominated by large clusters.
Halogen bonding has emerged as an important noncovalent interaction in a myriad of applications, including drug design, supramolecular assembly, and catalysis. Current understanding of the halogen bond is informed by electronic structure calculations on isolated molecules and/or crystal structures that are not readily transferable to liquids and disordered phases. To address this issue, we present a first-principles simulation-based approach for quantifying halogen bonds in molecular systems rooted in an understanding of nuclei-nuclei and electron-nuclei spatial correlations. We then demonstrate how this approach can be used to quantify the structure and dynamics of halogen bonds in condensed phases, using solid and liquid molecular chlorine as prototypical examples with high concentrations of halogen bonds. We close with a discussion of how the knowledge generated by our first-principles approach may inform the development of classical empirical models, with a consistent representation of halogen bonding.
Chemical polarity governs various mechanical, chemical and thermodynamic properties of dielectrics. Polar liquids have been amply studied, yet the basic mechanisms underpinning their dielectric properties remain not fully understood, as standard models following Debyes phenomenological approach do not account for quantum effects and cannot aptly reproduce the full dc-up-to-THz spectral range. Here, using the illustrative case of monohydric alcohols, we show that deep tunneling and the consequent intermolecular separation of excess protons and proton-holes in the polar liquids govern their static and dynamic dielectric properties on the same footing. We performed systematic ultrabroadband (0-10 THz) spectroscopy experiments with monohydric alcohols of different (0.4-1.6 nm) molecular lengths, and show that the finite lifetime of molecular species, and the proton-hole correlation length are the two principle parameters responsible for the dielectric response of all the studied alcohols across the entire frequency range. Our results demonstrate that a quantum non-rotational intermolecular mechanism drives the polarization in alcohols while the rotational mechanism of molecular polarization plays a secondary role, manifesting itself in the sub-terahertz region only.
A wide class of binary-state dynamics on networks---including, for example, the voter model, the Bass diffusion model, and threshold models---can be described in terms of transition rates (spin-flip probabilities) that depend on the number of nearest neighbors in each of the two possible states. High-accuracy approximations for the emergent dynamics of such models on uncorrelated, infinite networks are given by recently-developed compartmental models or approximate master equations (AME). Pair approximations (PA) and mean-field theories can be systematically derived from the AME. We show that PA and AME solutions can coincide under certain circumstances, and numerical simulations confirm that PA is highly accurate in these cases. For monotone dynamics (where transitions out of one nodal state are impossible, e.g., SI disease-spread or Bass diffusion), PA and AME give identical results for the fraction of nodes in the infected (active) state for all time, provided the rate of infection depends linearly on the number of infected neighbors. In the more general non-monotone case, we derive a condition---that proves equivalent to a detailed balance condition on the dynamics---for PA and AME solutions to coincide in the limit $t to infty$. This permits bifurcation analysis, yielding explicit expressions for the critical (ferromagnetic/paramagnetic transition) point of such dynamics, closely analogous to the critical temperature of the Ising spin model. Finally, the AME for threshold models of propagation is shown to reduce to just two differential equations, and to give excellent agreement with numerical simulations. As part of this work, Octave/Matlab code for implementing and solving the differential equation systems is made available for download.
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