Second generation Car-Parrinello MD: application to the h-BN/Rh(111) nanomesh

Hexagonal boron nitride sp2 layers grown and supported on the Rh(111) metal surface attracted quite some interest thanks to the structural and electronic peculiarities of this quasi-2D system. The honeycomb regular corrugation is the key feature at the origin of several properties and applications in nanotechnology, e.g., the selective adsorption and functionalisation related to the modulation of the electronic structure. Atomistic simulations play an important role, since they can shed light on the nature of such a complex interface, providing resolution of details that cannot be achieved experimentally. However, the studies by electronic structure calculations have been mostly limited to static models of the optimized system. The sampling of configurations at finite temperature by ab-initio molecular dynamics requires significantly larger computational effort, and can become unfeasible for large scale and metallic models, as it is the case of h-BN/Rh(111). In this work, we employ a recently developed Car-Parrinello-like approach to overcome the performance limitations of the standard Born-Oppenheimer molecular dynamics scheme, thus obtaining a speed-up of 17×. We report on the setup and the application of this approach to simulate the h-BN/Rh(111) interface at different temperatures and discuss the thermal stability of the corrugated pattern

Influence of the exchange and correlation functional on the structure of amorphous InSb and In3SbTe2 compounds

We have investigated the structural, vibrational, and electronic properties of the amorphous phase of InSb and In3SbTe2 compounds of interest for applications in phase change non-volatile memories. Models of the amorphous phase have been generated by quenching from the melt by molecular dynamics simulations based on density functional theory. In particular, we have studied the dependence of the structural properties on the choice of the exchange-correlation functional. It turns out that the use of the Becke-Lee-Yang-Parr functional provides models with a much larger fraction of In atoms in a tetrahedral bonding geometry with respect to previous results obtained with the most commonly used Perdew-Becke-Ernzerhof functional. This outcome is at odd with the properties of Ge2Sb2Te5 phase change compound for which the two exchange-correlation functionals yield very similar results on the structure of the amorphous phase

Western Star (Corner Brook, N.L.), 1914-05-13

The Western Star began publication on Newfoundland's west coast on 4 April 1900, appearing weekly with brief semiweekly periods up to 1952, when it became a daily. The current collection contains 21 April 1900 - 31 March 1926

On the role of non-spherical cavities in short length-scale density fluctuations in water

Density fluctuations in liquid water are at the heart of numerous phenomena associated with hydrophobic effects such as protein folding and the interaction between biomolecules. One of the most fundamental processes in this regard is the solvation of hydrophobic solutes in water. The vast majority of theoretical and numerical studies examine density fluctuations at the short length scale focusing exclusively on spherical cavities. In this work, we use both first-principles and classical molecular dynamics simulations to demonstrate that density fluctuations in liquid water can deviate significantly from the canonical spherical shapes. We show that regions of empty space are frequently characterized by exotic, highly asymmetric shapes that can be quite delocalized over the hydrogen bond network. Interestingly, density fluctuations of these shapes are characterized by Gaussian statistics with larger fluctuations. An important consequence of this is that the work required to create non spherical cavities can be substantially smaller than that of spheres. This feature is also qualitatively captured by the Lum-Chandler-Weeks theory. The scaling behavior of the free energy as a function of the volume at short length scales is qualitatively different for the nonspherical entities. We also demonstrate that nonspherical density fluctuations are important for accommodating the hydrophobic amino acid alanine and are thus likely to have significant implications when it comes to solvating highly asymmetrical species such as alkanes, polymers, or biomolecules

Fast Crystallization of the Phase Change Compound GeTe by Large-Scale Molecular Dynamics Simulations

Phase change materials are of great interest as active layers in rewritable optical disks and novel electronic nonvolatile memories. These applications rest on a fast and reversible transformation between the amorphous and crystalline phases upon heating, taking place on the nanosecond time scale. In this work, we investigate the microscopic origin of the fast crystallization process by means of large-scale molecular dynamics simulations of the phase change compound GeTe. To this end, we use an interatomic potential generated from a Neural Network fitting of a large database of ab initio energies. We demonstrate that in the temperature range of the programming protocols of the electronic memories (500-700 K), nucleation of the crystal in the supercooled liquid is not rate-limiting. In this temperature range, the growth of supercritical nuclei is very fast because of a large atomic mobility, which is, in turn, the consequence of the high fragility of the supercooled liquid and the associated breakdown of the Stokes-Einstein relation between viscosity and diffusivity