Marco Bernasconi

AB INITIO INFRARED SPECTRUM OF LIQUID WATER

Abstract An ab initio calculation of the infrared spectrum of liquid water has been performed using Car-Parrinello molecular dynamics and evaluating the electronic polarization by means of the Berry phase formulation. The major features of the spectrum are in good agreement with experiments and are shown to arise from specific vibrational motions of the water molecules. The effect of quantum corrections to the spectrum is discussed.

Coexistence of tetrahedral-and octahedral-like sites in amorphous phase change materials

Chalcogenide alloys are materials of interest for optical recording and nonvolatile memories. We perform ab initio molecular dynamics simulations aiming at shading light onto the structure of amorphous Ge2Sb2Te5 (GST), the prototypical material in this class. First principles simulations show that amorphous GST obtained by quenching from the liquid phase displays two types of short range order. One third of Ge atoms are in a tetrahedral environment while the remaining Ge, Sb, and Te atoms display a defective octahedral environment, reminiscent of cubic crystalline GST.

AB INITIO MOLECULAR-DYNAMICS SIMULATION OF K+ SOLVATION IN WATER

Potassium ion in water plays a very important role in chemistry and biology. In this paper, we study the solvation of this important ion using ab initio Car–Parrinello molecular dynamics. We work within the pseudopotential, density-functional approach and use the BLYP (Becke–Lee–Yang– Parr) generalized gradient approximation to the exchange and correlation potential. An analysis of the structural properties of the solvation shell shows good agreement with existing experiments, as well as with previous simulations based on classical potentials.

Structural Phase Transformations via First-Principles Simulation

We present a new simulation scheme for structural phase transitions via first-principles molecular dynamics. The method is obtained by combining the Car-Parrinello method for ab initio simulation with the Parrinello-Rahman method to account for variable cell shape. We demonstrate the validity of our approach by simulating the spontaneous transformation of silicon from diamond to simple hexagonal phase under high pressure.

First-principles study of crystalline and amorphous Ge2Sb2Te5 and the effects of stoichiometric defects

Based on ab initio molecular dynamics simulations, we investigated the structural, electronic and vibrational properties of cubic and amorphous Ge(2)Sb(2)Te(5) (GST) phase change material, focusing in particular on the effects of defects in stoichiometry on the electronic properties. It turned out Ge/Sb deficiencies (excess) in the cubic phase induce a shift of the Fermi level inside the valence (conduction) bands. In contrast, the amorphous network is flexible enough to accommodate defects in stoichiometry, keeping the Fermi level pinned at the center of the bandgap (at zero temperature). Changes in the structural and electronic properties induced by the use of hybrid functionals (HSE03, PBE0) instead of gradient corrected functionals (PBE) are addressed as well. Analysis of vibrational spectra and Debye-Waller factors of cubic and amorphous GST is also presented.

Simulation of structural phase transitions by metadynamics

Abstract We describe here in detail the recently introduced methodology for simulation of structural transitions in crystals. The applications of the new scheme are illus trated on various kinds of crystals and the advantages with respect to previous schemes are emphasized. The relevance of the new method for the problem of crystal structure prediction is also discussed.

Density-functional study of hydration of sodium in water clusters

The structures and hydration energies of small water clusters containing a sodium atom or ion are investigated within density-functional theory by using the Becke–Lee–Yang–Parr (BLYP) generalized gradient corrections to the local density approximation exchange and correlation energy, norm-conserving pseudopotentials, and a plane-wave expansion of Kohn–Sham orbitals. The hydration energies obtained for both neutral and ionized clusters, as well as the ionization potentials, are in good agreement with experiment and with available quantum-chemical calculations.

Ab initio simulation of water interaction with the (100) surface of pyrite

Car–Parrinello simulations have been performed to study the interaction of water with pyrite (100) surface. The stability and the structural and electronic properties of both the molecular and dissociative adsorptions have been addressed. We found a very strong preference for molecular adsorption on the surface iron sites, in agreement with experiment. The dissociative chemisorption of water is energetically disfavored and is even locally unstable; the dissociated fragments transform back to the stable molecular form in a short molecular dynamics run. The calculations revealed that hydrogen bonding plays an important role in the stabilization of the adsorbed water for both the molecular and the dissociative states. We have shown that water forms a coordinative covalent bond with the surface iron atoms by donating electron to the empty iron dz2 orbitals which are the lowest empty states on the clean surface. At full coverage, the sulfur 3p states thus become the lowest available empty states and therefore ...

First-principle-constant pressure molecular dynamics

Abstract We present a new method for first-principles numerical simulation of solid-solid phase transformation. The method is applied to the study of pressure induced transformations in silicon and carbon.

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.