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Due to their potential role in the peculiar geophysical properties of the ice giants Neptune and Uranus, there has been a growing interest in superionic (SI) phases of water ice. So far, however, little attention has been given to their mechanical properties, even though plastic deformation processes in the interiors of planets are known to affect long-term processes, such as plate tectonics and mantle convection. Here, using density functional theory calculations and machine learning techniques, we assess the mechanical response of high-pressure/temperature solid phases of water in terms of their ideal shear strength (ISS) and dislocation behavior. The ISS results are well described by the renormalized Frenkel model of ideal strength and indicate that the SI ices are expected to be highly ductile. This is further supported by deep neural network molecular dynamics simulations for the behavior of lattice dislocations for the SI face-centered cubic (fcc) phase. Dislocation velocity data indicate effective shear viscosities that are orders of magnitude smaller than that of Earth’s lower mantle, suggesting that the plastic flow of the internal icy layers in Neptune and Uranus may be significantly faster than previously foreseen.

Using nonequilibrium molecular dynamics simulations, we investigate the process of grain-boundary (GB) sliding in ice Ih. We focus on the ∑35 symmetric tilt boundary, which has been observed experi...

The anomalous increase in compressibility and heat capacity of supercooled water has been attributed to its structural transformation of into a four-coordinated liquid. Experiments revealed that κ

This article describes nonequilibrium techniques for the calculation of free energies of solids using molecular dynamics (MD) simulations. These methods provide an alternative to standard equilibrium thermodynamic integration methods and often present superior efficiency. Here we describe the implementation in the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code of two specific nonequilibrium processes that allow the calculation of the free-energy difference between two different system Hamiltonians as well as the free-energy temperature dependence of a given Hamiltonian, respectively. The theory behind the methods is summarized, and we describe (including fragments of LAMMPS scripts) how the process parameters should be selected to obtain the best-possible efficiency in the calculations of free energies using nonequilibrium MD simulations. As an example of the application of the methods we present results related to polymorphic transitions for a classical potential model of iron.

Agradecimentos: We acknowledge support from FAPESP Grants No. 2013/08293-7 and 2016/23891-6, CNPq. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. The authors acknowledge the National Laboratory for Scientific Computing (LNCC/MCTI, Brazil) for providing HPC resources of the SDumont supercomputer, URL: http://sdumont.lncc.br Abstract: We present a guide to compute the absolute free energies of classical fluids using nonequilibrium free-energy techniques within the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code. The main approach is based on the construction of a thermodynamic path connecting the fluid of interest to either atomic or molecular variants of the Uhlenbeck-Ford (UF) model as reference systems. We describe these reference systems in detail, discuss their implementation in the LAMMPS package and make available source code, scripts as well as auxiliary files. As an illustration we detail a number of distinct applications, involving systems characterized by fundamentally different interactions. In addition to two different atomic models (mW water and the MEAM-2NN CuZr liquid binary alloy), we consider three molecular models for water, two of them rigid (TIP4P and SPC/E) and one flexible (q-SPC/Fw). For the molecular models we develop UF-based reference systems for which the free energies are given by a sum of two contributions: an intermolecular part described by the known UF free energy and an intramolecular contribution that can be determined analytically. The tools described in this paper provide a platform on which fluid-phase free energies can be easily and efficiently computed using the LAMMPS code. In addition to being useful for the development of new models for liquid phases, the tools may also find applications in the construction of community databases containing thermodynamic properties of existing models CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP Fechado

In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid–liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water’s peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility–viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed. Graphic abstract: [Figure not available: see fulltext.].

Agradecimentos: We gratefully acknowledge support from the Brazilian agencies CNPq, Capes, Fapesp (Grant No. 2016/23891-6) and the Center for Computing in Engineering and Sciences—Fapesp/Cepid (Grant No. 2013/08293-7). Part of the calculations were performed at CCJDR-IFGW-UNICAMP. The authors acknowledge the National Laboratory for Scientific Computing (LNCC/MCTI, Brazil) for providing HPC resources of the SDumont supercomputer, which have contributed to the research results reported in this paper. URL: http://sdumont.lncc.br. We thank Alexander Stukowksi and Peter Larsen for their assistance with Ovito’s grain segmentation algorithm Abstract: We consider rapid cooling processes in classical, three-dimensional, purely repulsive binary mixtures in which an initial infinite-temperature (ideal-gas) configuration is instantly quenched to zero temperature. It is found that such systems display two kinds of ordering processes, the type of which can be controlled by tuning the interactions between unlike particles. While strong inter-species repulsion leads to chemical ordering in terms of an unmixing process, weak repulsion gives rise to spontaneous crystallization, maintaining chemical homogeneity. This result indicates the existence of a transition in the topography of the underlying potential-energy landscape as the intra-species interaction strength is varied. Furthermore, the dual-type behavior appears to be universal for repulsive pair-interaction potential-energy functions in general, with the propensity for the crystallization process being related to their behavior in the neighborhood of zero separation CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP Fechado

Using molecular dynamics simulations, we study the nanoindentation response of the ice Ih basal surface using two popular water models, namely, the all-atom TIP4P/Ice potential and the coarse-grained mW model. In particular, we consider two markedly different temperatures at which a quasi-liquid layer (QLL) is or is not present. We discuss loading curves, hardness estimates, deformation mechanisms, and residual imprints, considering the effect of the QLL, indenter size, and penetration rate. At very low temperatures, in the absence of a QLL, both potentials produce similar loading curves and deformation mechanisms. Close to the melting temperature, however, important differences were found, including deviations in the QLL thickness and fraction as well as the presence of a competition between pressure-induced melting and recrystallization events. Nevertheless, both potentials exhibit similar deformation mechanisms and steady-state hardness estimates that are consistent with experimental data. In addition to contributing to the discussion regarding the interpretation of experimental AFM loading curves, the present results provide valuable information concerning the simulation of contact problems involving ice and the behavior of these two popular water models under such circumstances. Fil: Santos Flórez, Pedro Antonio. Universidade Estadual de Campinas; Brasil Fil: Ruestes, Carlos Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Interdisciplinario de Ciencias Básicas. - Universidad Nacional de Cuyo. Instituto Interdisciplinario de Ciencias Básicas; Argentina Fil: De Koning, Maurice. Universidade Estadual de Campinas; Brasil

We apply a set of machine-learning (ML) techniques for the global exploration of the phase diagrams of two frustrated 2D Ising models with competing interactions. Based on raw Monte Carlo spin configurations generated for random system parameters, we apply principal-component analysis (PCA) and auto-encoders to achieve dimensionality reduction, followed by clustering using the DBSCAN method and a support-vector machine classifier to construct the transition lines between the distinct phases in both models. The results are in very good agreement with available exact solutions, with the auto-encoders leading to quantitatively superior estimates, even for a data set containing only 1400 spin configurations. In addition, the results suggest the existence of a relationship between the structure of the optimized auto-encoder latent space and physical characteristics of both systems. This indicates that the employed approach can be useful in perceiving fundamental properties of physical systems in situations where a priori theoretical insight is unavailable.

Using ab initio and classical molecular dynamics simulations, we study pre-melting phenomena in pristine coincident-site-lattice grain boundaries (GBs) in proton-disordered hexagonal ice Ih at temperatures just below the melting point Tm. Concerning pre-melt-layer thicknesses, the results are consistent with the available experimental estimates for low-disorder impurity-free GBs. With regard to molecular mobility, the simulations provide a key new insight: the translational motion of the water molecules is found to be subdiffusive for time scales from ∼10 ns up to at least 0.1 μs. Moreover, the fact that the anomalous diffusion occurs even at temperatures just below Tm where the bulk supercooled liquid still diffuses normally suggests that it is related to the confinement of the GB pre-melt layers by the surrounding crystalline environment. Furthermore, we show that this behavior can be characterized by continuous-time random walk models in which the waiting-time distributions decay according to power-laws that are very similar to those describing dynamics in glass-forming systems.

Using molecular dynamics simulations, we compute the elastic constants of ice I

The water-solvated excess electron (EE) is a key chemical agent whose hallmark signature, its asymmetric optical absorption spectrum, continues to be a topic of debate. While nearly all investigation has focused on the liquid-water solvent, the fact that the crystalline-water solvated EE shows a very similar visible absorption pattern has remained largely unexplored. Here, we present spin-polarized density-functional theory calculations subject to periodic boundary conditions of the interplay between an EE and a number of intrinsic lattice defects in ice Ih. Our results show that the optical absorption signatures in the presence of three unsaturated hydrogen bonds (HB) are very similar to those observed experimentally. Its low-energy side can be attributed to transitions between the EE ground state and a single localized excited level, in a picture that is different from that for the liquid solvent, where this portion has been associated with hydrogen-like s → p excitations. The blue tail, on the other hand, relates to transitions between the EE ground state and delocalized excited states, which is in line with the bound-to-continuum transition interpretations for the EE in liquid water. Finally, we find that, depending on the number of dangling HBs participating in the EE trap, its charge density may spontaneously break the spin degeneracy through exchange interactions with the surrounding electrons, displaying the many-electron quantum nature of the EE problem in ice Ih.

In this paper, we present an overview of crystal imperfections in ice Ih. Due to its molecular nature, the fundamental asymmetry of the hydrogen bond, and proton disorder, crystal defects in this condensed form of water reveal a complexity not usually seen in atomic crystalline solids. The discussion is organized in terms of the spatial extent of the defects. We start with zero-dimensional imperfections such as the molecular vacancy and interstitial, Bjerrum, and ionic defects, as well as possible defect complexes that can be formed from them. Subsequently, we turn to the properties of dislocations, which are the one-dimensional disturbances that carry plastic deformation in crystalline solids. Finally, we discuss two-dimensional defects such as stacking faults and grain boundaries and discuss to what extent the latter are similar to other interfaces in ice Ih such as the free surface. We conclude with an outlook at the road ahead, discussing future challenges toward understanding the role of crystal defects in the macroscopic behavior of ice Ih.

A multiscale approach to investigate the influence of the grain boundary (GB) lattice structures on the dynamic intergranular failure in 3 D polycrystalline materials is proposed. The model comprises the meso- and atomistic scales using the boundary element method (BEM) and molecular dynamics (MD), respectively. At the mesoscale, stochastic grain morphologies, random crystalline orientations and initial defects are included in the physical model. Moreover, a dynamic high-rate load is imposed to produce dynamic stress and strain waves propagating throughout the polycrystal, inducing the material to be susceptible to fail. The intergranular failure is governed by the critical energy density for shear and cleavage modes, evaluated from a set of nano GBs at the atomistic scale. The novelty is the assessment of the energy density considering its dependency on the interface lattice, leading to a group of failure criteria distributed along the aggregate. The difference in the order of magnitude between these length scales is a challenge for the transition multiscale model. Hence, an asymptotic scaling methodology is adapted for bridging the mechanical strength. Finally, it is worth noting that the level of detail of this criterion, is a remarkable enhancement over other intergranular failure models.

We assess the elastic stiffness constants of hexagonal proton-disordered ice Ih as described by density-functional theory calculations. Specifically, we compare the results for a set of nine exchange-correlation functionals, including standard generalized-gradient approximations (GGAs), the strongly constrained and appropriately normed (SCAN) metaGGA functional, and a number of dispersion-corrected versions based on the van der Waals (vdW) and VV10 schemes. Compared to the experimental data, all functionals predict an excessively stiff response to tensile and compressive distortions, as well as shear deformations along the basal plane, with the SCAN metaGGA functional displaying the largest deviations as compared to the experimental values. These discrepancies are found to correlate with underestimates of inter-molecular distances, on the one hand, and overestimates of intra-molecular separations, on the other. The inclusion of non-local vdW corrections according to the vdW approach generally improves these structural parameters and softens the elastic response functions compared to their parent GGA functionals. The dispersion-corrected SCAN-rVV10 functional, however, acts in the opposite direction, further worsening the comparison to experiment. In this view, it appears useful that the database employed to gauge the quality of exchange-correlation functionals for water includes an assessment of their elastic response of ice Ih and possibly other crystalline phases.

Using molecular dynamics simulations, we assess the uniaxial deformation response of ice I

We discuss the role of nuclear quantum fluctuations in ice Ih, focusing on the hydrogen-bond (HB) structure and the molecular dipole-moment distribution. For this purpose we carry out DFT-based first-principles molecular dynamics and path-integral molecular dynamics simulations at T = 100 K. We analyze the HB structure in terms of a set of parameters previously employed to characterize molecular structures in the liquid phase and compute the molecular dipole moments using the maximally-localized Wannier functions. The results show that the protons experience very large digressions driven by quantum fluctuations, accompanied by major rearrangements in the electronic density. As a result of these protonic quantum fluctuations the molecular dipole-moment distribution is substantially broadened as well as shifted to a larger mean value when compared to the results obtained when such fluctuations are neglected. In terms of dielectric constants, the reconciliation between the greater mean dipole moment and experimental indications that the dielectric constant of H2O ice is lower than that of D2O ice would indicate that the topology of the HB network is sensitive to protonic quantum fluctuations.

We investigate the uptake of HCl and HF at lattice vacancies in ice Ih as a function of their distance to the basal-plane surface layer using density-functional theory calculations. The results for HCl display large dispersions in the binding-energy results due to the appearance of distinct dissociation states. The layer-averaged results suggest that the uptake of HCl is most favorable in the two layers just below the surface, which is consistent with available experimental indications. The behavior of HF is found to be manifestly different due to the fact that it is a weaker acid. The dispersion in the binding-energy values is significantly less compared to the case of HCl, and the average values are essentially equal to the bulk value, regardless of layer position. This suggests that, in contrast to the case of HCl, there should not be any tendency for accumulation of HF near the surface.

The Uhlenbeck-Ford (UF) model was originally proposed for the theoretical study of imperfect gases, given that all its virial coefficients can be evaluated exactly, in principle. Here, in addition to computing the previously unknown coefficients B11 through B13, we assess its applicability as a reference system in fluid-phase free-energy calculations using molecular simulation techniques. Our results demonstrate that, although the UF model itself is too soft, appropriately scaled Uhlenbeck- Ford (sUF) models provide robust reference systems that allow accurate fluid-phase free-energy calculations without the need for an intermediate reference model. Indeed, in addition to the accuracy with which their free energies are known and their convenient scaling properties, the fluid is the only thermodynamically stable phase for a wide range of sUF models. This set of favorable properties may potentially put the sUF fluid-phase reference systems on par with the standard role that harmonic and Einstein solids play as reference systems for solid-phase free-energy calculations.

The elastic constants of hcp $^{4}$He are computed using the path-integral Monte Carlo (PIMC) method. The stiffness coefficients are obtained by imposing different distortions to a periodic cell containing 180 atoms, followed by measurement of the elements of the corresponding stress tensor. For this purpose an appropriate path-integral expression for the stress tensor observable is derived and implemented into the PIMC++ package. In addition to allowing the determination of the elastic stiffness constants, this development also opens the way to an explicit atomistic determination of the Peierls stress for dislocation motion using the PIMC technique. A comparison of the results to available experimental data shows an overall good agreement of the density dependence of the elastic constants, with the single exception of C$_{13}$. Additional calculations for the bcc phase, on the other hand, show good agreement for \textit{all} elastic constants., 3 figures

Using path-integral Monte Carlo simulations, we assess the core structure and mobility of the screw and edge basal-plane dislocations in hcp $^{4}\mathrm{He}$. Our findings provide key insights into recent interpretations of giant plasticity and mass flow junction experiments. First, both dislocations are dissociated into nonsuperfluid Shockley partial dislocations separated by ribbons of stacking fault, suggesting that they are unlikely to act as one-dimensional channels that may display L\"uttinger-liquid-like behavior. Second, the centroid positions of the partial cores are found to fluctuate substantially, even in the absence of applied shear stresses. This implies that the lattice resistance to motion of the partial dislocations is negligible, consistent with the recent experimental observations of giant plasticity. Further results indicate that both the structure of the partial cores and the zero-point fluctuations play a role in this extreme mobility.

Using molecular dynamics simulations, we assess the uniaxial deformation response of ice Ih as described by two popular water models, namely, the all-atom TIP4P/Ice potential and the coarse-grained mW model. In particular, we investigate the response to both tensile and compressive uniaxial deformations along the [0001] and [01̄10] crystallographic directions for a series of different temperatures. We classify the respective failure mechanisms and assess their sensitivity to strain rate and cell size. While the TIP4P/Ice model fails by either brittle cleavage under tension at low temperatures or large-scale amorphization/melting, the mW potential behaves in a much more ductile manner, displaying numerous cases in which stress relief involves the nucleation and subsequent activity of lattice dislocations. Indeed, the fact that mW behaves in such a malleable manner even at strain rates that are substantially higher than those applied in typical experiments indicates that the mW description of ice Ih is excessively ductile. One possible contribution to this enhanced malleability is the absence of explicit protons in the mW model, disregarding the fundamental asymmetry of the hydrogen bond that plays an important role in the nucleation and motion of lattice dislocations in ice Ih. Fil: Santos Flórez, Pedro Antonio. Universidade Estadual de Campinas; Brasil Fil: Ruestes, Carlos Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina Fil: de Koning, Maurice. Universidade Estadual de Campinas; Brasil

We investigate the influence of anharmonic effects on the zero-point vacancy concentration in a boson system model in the solid phase at T=0 K. We apply the reversible-work method to compute the vacancy formation free energy and the vacancy concentration in the system. A comparison of our results with those obtained using the harmonic approximation show that anharmonic effects reduce the formation free energy by ∼25%, leading to an increase of the zero-point vacancy concentration by more than an order of magnitude.

We present a density-function theory (DFT) study of Bjerrum-defect trapping centers involving the molecular vacancy in ice Ih. As a first step, we compute the intrinsic migration barrier to D-defect motion using the nudged elastic band (NEB) method and find them to be of the same order of magnitude as the energy barriers involving intrinsic L-defect motion. This finding suggests that intrinsic mobility factors cannot explain the experimentally observed inactivity of D defects, supporting the idea that D defects are trapped at other lattice-defect sites. Next we study the defect complexes formed by the combination of isolated D and L defects with a molecular vacancy. The corresponding geometries show that the formation of these aggregates significantly reduces elastic distortions that are present in isolated Bjerrum defects. An analysis of the energetics involved in the formation of both defect complexes reveals a significant binding energy, indicating that the molecular vacancy represents a strong trapping center for Bjerrum defects. On the other hand, the fact that there is no difference between the absolute values of the binding energies for both D and L defects suggests that the vacancy affects both species of Bjerrum defects in a similar fashion, possibly ruling out the vacancy trapping centers as an explanation for the experimentally observed inactivity of D defects.

We investigate the resistance on the glide of lattice dislocations between adjacent crystal grains due to the presence of a grain boundary (GB). Applying a combination of molecular dynamics (MD) simulations and a line tension (LT) model we identify the geometrical parameters that are relevant in the description of this process. In the MD simulations we observe slip transmission of dislocation loops nucleated from a crack tip near a series of pure tilt GBs in Ni. The results are rationalized in terms of a LT model for the activation of a Frank-Read source in the presence of a GB. It is found that the slip transmission resistance is a function of only three variables: firstly, the ratio of resolved stress on the incoming slip system to that on the outgoing slip system, secondly, the magnitude of any residual Burgers vector content left in the GB and, thirdly, the angle between the traces of the incoming and outgoing slip planes in the GB plane. Comparison with the MD simulations and experimental da...

Using path-integral Monte Carlo simulations, we compute the ideal shear strength (ISS) on the basal plane of hcp (4)He. The failure mode upon reaching the ISS limit is characterized by the homogeneous nucleation of a stacking fault and it is found to be anisotropic, consistent with Schmid's law of resolved shear stress. Comparing the ISS of hcp (4)He to a large set of classical crystals shows that it closely fits the approximately universal modified Frenkel model of ideal strength. In addition to giving quantitative stress levels for the homogeneous nucleation of extended defects in hcp (4)He, our findings lend support to assumptions in the literature that inherently classical models remain useful for the description of mechanical behavior in quantum crystals.

We present a dynamic implementation of the Clausius–Clapeyron integration (CCI) method for mapping out phase-coexistence boundaries through a single atomistic simulation run. In contrast to previous implementations, where the reversible path of coexistence conditions is generated from a series of independent equilibrium simulations, dynamic Clausius–Clapeyron integration (d-CCI) explores an entire coexistence boundary in a single nonequilibrium simulation. The method gives accurately the melting curve for a system of particles interacting through the Lennard-Jones potential. Furthermore, we apply d-CCI to compute the melting curve of an ab initio pair potential for argon and verify earlier studies on the effects of many-body interactions and quantum effects in the melting of argon. The d-CCI method shows to be effective in both applications, giving converged coexistence curves spanning a wide range of thermodynamic states from relatively short nonequilibrium simulations.

We investigated the interaction of intrinsic point defects (vacancies and self-interstitials) with a partial dislocation in silicon. Using a combination of zero-temperature ab initio total energy calculations with finite temperature free-energy calculations based on an interatomic potential, we obtained energies for the relevant core defects. The formation energies of vacancies and interstitials in the core of a 30° glide partial dislocation are considerably lower (by more than 1 eV) than in the bulk. However, even at high temperatures, the predicted thermal concentration of shuffle segments, comprised of a row of vacancies or interstitials in the core, is low. Therefore, in the competition between two alternative positions (shuffle or glide {1 1 1} plane sub-sets) for the core of a 30° partial dislocation, the glide segment is strongly favored.

We present a method, for highly efficient free-energy calculations by means of molecular dynamics and Monte Carlo simulations, which is an optimized combination of coupling parameter and adiabatic switching formalisms. This approach involves dynamical reversible scaling of the potential energy function of a system of interest, and allows accurate determination of its free energy over a wide temperature interval from a single simulation. The method is demonstrated in two applications: crystalline Si at zero pressure and a fcc nearest-neighbor antiferromagnetic Ising model.

Agradecimentos: We gratefully acknowledge support from the Brazilian agencies CNPq, Fapesp, Capes and the Center for Computational Engineering and Sciences-Fapesp/Cepid #2013/08293-7. Part of the calculations were performed at CCJDR-IFGW-UNICAMP and CENAPAD-SP. We would like to thank the anonymous reviewers for their helpful suggestions Abstract: Using molecular dynamics simulations we analyze the dynamics of two atomic liquids that display a liquid-liquid phase transition (LLPT): Si described by the Stillinger-Weber potential and Ga as modeled by the modified embedded-atom model. In particular, our objective is to investigate the extent to which the presence of a dip in the self-intermediate scattering function is a manifestation of an excess of vibrational states at low frequencies and may be associated with a fragile-to-strong transition (FTST) across the LLPT, as suggested recently. Our results suggest a somewhat different picture. First, in the case of Ga we observe the appearance of an excess of vibrational states at low frequencies, even in the absence of the appearance of a dip in the self-intermediate scattering function across the LLPT. Second, studying the behavior of the shear viscosities traversing the LLPTs we find that both substances are fragile in character above and below their respective LLPT temperatures. Instead of a FTST in an absolute sense these findings are more in line with a view in which the LLPTs are accompanied by a transition from a more fragile to a less fragile liquid. Furthermore, we do not find this transition to correlate with the presence of a dip in the intermediate scattering function CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado

We consider the molecular structure and energetics of extended defects in proton-disordered hexagonal ice I${}_{h}$. Using plane-wave density functional theory (DFT) calculations, we compute the energetics of stacking faults and determine the structure of the ${30}^{\ensuremath{\circ}}$ and ${90}^{\ensuremath{\circ}}$ partial dislocations on the basal plane. Consistent with experimental data, the formation energies of all fully reconstructed stacking faults are found to be very low. This is consistent with the idea that basal-plane glide dislocations in ice I${}_{h}$ are dissociated into partial dislocations separated by an area of stacking fault. For both types of partial dislocation we find a strong tendency toward core reconstruction through pairwise hydrogen-bond reformation. In the case of the ${30}^{\ensuremath{\circ}}$ dislocation, the pairwise hydrogen-bond formation leads to a period-doubling core structure equivalent to that seen in zinc-blende semiconductor crystals. For the ${90}^{\ensuremath{\circ}}$ partial we consider two possible core reconstructions, one in which the periodicity of the structure along the core remains unaltered and another in which it is doubled. The latter is preferred, although the energy difference between both is rather small, so that a coexistence of both reconstructions appears plausible. Our results imply that a mobility theory for dislocations on the basal plane in ice I${}_{h}$ should be based on the idea of reconstructed partial dislocations.

A Comment on the Letter by Hichem Dammak et al., Phys. Rev. Lett. 103, 190601 (2009). The authors of the Letter offer a Reply.

We investigate the structure and mobility of dislocations in hcp $^{4}\mathrm{He}$ crystals. In addition to fully characterizing the five elastic constants of this system, we obtain direct insight into dislocation core structures on the basal plane, which demonstrates a tendency toward dissociation into partial dislocations. Moreover, our results suggest that intrinsic lattice resistance is an essential factor in the mobility of these dislocations. This insight sheds new light on the possible correlation between dislocation mobility and the observed macroscopic behavior of crystalline $^{4}\mathrm{He}$.

We investigate the structure and mobility of dislocations in hcp 4He crystals. In addition to fully characterizing the five elastic constants of this system, we obtain direct insight into dislocation core structures on the basal plane, which demonstrates a tendency toward dissociation into partial dislocations. Moreover, our results suggest that intrinsic lattice resistance is an essential factor in the mobility of these dislocations. This insight sheds new light on the possible correlation between dislocation mobility and the observed macroscopic behavior of crystalline 4He.

Agradecimentos: The work was supported by the Brazilian agencies FAPESP, CNPq, and CAPES. Part of the computations was carried out at CENAPAD-SP Abstract: We report on theoretical results that lend support to recent experimental observations suggesting the existence of a first-order liquid-liquid phase transformation (LLPT) in gallium. Using molecular dynamics simulation based on a modified embedded-atom model, we observe a transition from a high-density to a low-density liquid in the supercooled regime. The first-order character of the transition is established through the detection of the release of latent heat and our findings suggest that the LLPT terminates in a critical point that is located in the tensile-strained domain of the metastable phase diagram FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Aberto

We discuss a number of examples that demonstrate the value of computational modeling as a complementary approach in the physics and chemistry of ice I h , where real-life experiments often do not give direct access to the desired information or whose interpretation typically requires uncontrollable assumptions. Specifically, we discuss two cases in which, guided by experimental insight, density-functional-theory-based first-principles methods are applied to study the properties of lattice defects and their relationship to ice I h s macroscopic behavior. First, we address a question involving molecular point defects, examining the energetics of formation of the molecular vacancy and a number of different molecular interstitial configurations. The results indicate that, as suggested by earlier experiments, a configuration involving bonding to the surrounding hydrogen-bond network is the preferred interstitial structure in ice I h . The second example involves the application of modeling to elucidate on the microscopic origin of the experimental observation that a specific type of ice defect is effectively immobile while others are not. Inspired by previous suggestions that this defect type may be held trapped at other defect sites and our finding that the bound configuration is the preferred interstitial configuration in ice I h , we use first-principles modeling to examine the binding energetics of the specific ice defect to the molecular vacancy and interstitial. The results suggest a preferential binding of the immobile defect to the molecular interstitial, possibly explaining its experimentally observed inactivity.

We present a model for the determination of the thermal equilibrium concentrations of Bjerrum defects, molecular point defects, and their aggregates in ice Ih. First, using a procedure which minimizes the free energy of an ice crystal with respect to the numbers of defect species, we derive a set of equations for the equilibrium concentrations of free Bjerrum and point defects, as well their complexes. Using density-functional-theory calculations, we then evaluate the binding energies of Bjerrum-defect/vacancy and Bjerrum-defect/interstitial complexes. In contrast to the complexes which involve the molecular vacancy, the results suggest that the molecular interstitial binds preferentially to the D-type Bjerrum defect. Using both theoretical binding and formation free energies as well as the available experimental data, we find that the preferential binding and the substantial presence of the interstitial as the predominant point defect in ice Ih may lead to conditions in which the number of free D defects becomes considerably smaller than that of free L defects. Such a scenario could possibly be involved in the experimentally observed inactivity of D-type Bjerrum defects in the electrical properties of ice Ih FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado

We present a first-principles study of the molecular vacancy and three distinct molecular interstitial structures in ice ${\mathrm{I}}_{h}$. The results indicate that, due to its bonding to the surrounding hydrogen-bond network, the bond-center (Bc) configuration is the favored molecular interstitial in ice ${\mathrm{I}}_{h}$. A comparison between the vacancy and the Bc interstitial suggests that the former is the predominant molecular point defect for $T\ensuremath{\lesssim}200K$ although a crossover scenario in which the latter becomes favored below the melting point is conceivable.

The formation thermodynamics and migration properties of self-interstitials in aluminum and nickel are investigated as a function of temperature using atomistic simulation techniques and embedded-atom-type interatomic potentials. Molecular dynamics and nonequilibrium free-energy techniques are employed to investigate anharmonic effects on the ${H}_{O}$ $⟨100⟩$ dumbbell formation properties. The equilibrium concentration of this defect is compared to those of vacancies and divacancies. The results are then analyzed in the framework of the interstitialcy model, according to which very high vibrational formation entropies should be expected for self-interstitials at high temperatures. The kinetics of self-interstitial migration is also investigated using different atomistic techniques, revealing the simultaneous activity of more than one distinct interstitial configuration as the temperature increases.

We present a first-principles study of the structure and energetics of Bjerrum defects in ice Ih and compare the results to experimental electrical conductivity data. While the DFT result for the activation energy is in good agreement with experiment, we find that its two components have quite different values. Aside from providing new insight into the fundamental parameters of the microscopic electrical theory of ice, our results suggest the activity of traps in doped ice in the temperature regime typically assumed to be controlled by the free migration of L defects., Comment: 4 pages, 4 Figures, 1 Table

We consider the issue of optimizing linear-regime nonequilibrium simulations to estimate free-energy differences. In particular, we focus on the problem of finding the best-possible driving function lambda(t) that, for a given thermodynamic path, simulation algorithm, and amount of computational effort, minimizes dissipation. From the fluctuation-dissipation theorem it follows that, in the linear-response regime, the dissipation is controlled by the magnitude and characteristic correlation time of the equilibrium fluctuations in the driving force. As a result, the problem of finding the optimal switching scheme involves the solution of a standard problem in variational calculus: the minimization of a functional with respect to the switching function. In practice, the minimization involves solving the associated Euler-Lagrange equation subject to a set of boundary conditions. As a demonstration we apply the approach to the simple, yet illustrative problem of computing the free-energy difference between two classical harmonic oscillators with very different characteristic frequencies.

Stimulated by the progress of computer technology over the past decades, the field of computer simulation has evolved into a mature branch of modern scientific investigation. It has had a profound impact in many areas of research including condensed-matter physics, chemistry, materials and polymer science, as well as in biophysics and biochemistry. Many problems of interest in all of these areas involve complex many-body systems and analytical solutions are generally not available. In this light, atomistic simulations play a particularly important role, giving detailed insight into the fundamental microscopic processes that control the behavior of complex systems at the macroscopic level. They provide key and effective tools for providing ab initio predictions, interpreting complex experimental data, as well as conducting computational “experiments” that are difficult or impossible to realize in a laboratory.