Showing papers by "Roberto Car published in 2010"

X-ray absorption signatures of the molecular environment in water and ice.

Abstract: The x-ray absorption spectra of water and ice are calculated with a many-body approach for electron-hole excitations. The experimental features, including the effects of temperature change in the liquid, are reproduced from configurations generated by ab initio molecular dynamics. The spectral difference between the solid and the liquid is due to two major short-range order effects. One, due to breaking of hydrogen bonds, enhances the pre-edge intensity in the liquid. The other, due to a nonbonded molecular fraction in the first coordination shell, affects the main spectral edge in the conversion of ice to water. This effect may not involve hydrogen bond breaking as shown by experiment in high-density amorphous ice.

Displaced path integral formulation for the momentum distribution of quantum particles.

Abstract: The proton momentum distribution, accessible by deep inelastic neutron scattering, is a very sensitive probe of the potential of mean force experienced by the protons in hydrogen-bonded systems. In this work we introduce a novel estimator for the end-to-end distribution of the Feynman paths, i.e., the Fourier transform of the momentum distribution. In this formulation, free particle and environmental contributions factorize. Moreover, the environmental contribution has a natural analogy to a free energy surface in statistical mechanics, facilitating the interpretation of experiments. The new formulation is not only conceptually but also computationally advantageous. We illustrate the method with applications to an empirical water model, ab initio ice, and one dimensional model systems.

Displaced path integral formulation for the momentum distribution of quantum particles

Abstract: Lin Lin, Joseph A. Morrone,* Roberto Car, and Michele Parrinello Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey 08544, USA Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA Department of Physics, Princeton University, Princeton, New Jersey 08544, USA Computational Science, Department of Chemistry and Applied Biosciences, ETH Zurich, USI Campus, Via Giuseppe Buffi 12, CH-6900 Lugano, Switzerland (Received 6 April 2010; revised manuscript received 5 June 2010; published 9 September 2010)

Influence of asymmetry on bias behavior of spin torque

Abstract: We report calculations, based on the tight-binding model and the nonequilibrium Keldysh formalism, of the effect of band-filling (BF) asymmetry between the ferromagnetic leads on the bias behavior of the spin torque and the tunneling magnetoresistance (TMR) in magnetic tunnel junctions. The underlying mechanism for the asymmetry-induced change in the bias dependence of TMR and the spin-transfer component, ${T}_{\ensuremath{\parallel}}$, is the interplay of charge and spin currents in the ferromagnetic (FM) and antiferromagnetic (AF) configurations. The BF asymmetry has a dramatic effect on the low-bias behavior of the fieldlike component, ${T}_{\ensuremath{\perp}}$, which can vary from linear to quadratic bias dependence with positive or negative curvature, thus reconciling the apparently contradictory experimental results. A general expression is derived relating ${T}_{\ensuremath{\perp}}$ with four independent nonequilibrium interlayer exchange couplings (NEIECs), ${J}_{\text{FM}}^{\ensuremath{\sigma}{\ensuremath{\sigma}}^{\ensuremath{'}}}$, associated with the majority- and minority-spin channels, $\ensuremath{\sigma},{\ensuremath{\sigma}}^{\ensuremath{'}}=\ensuremath{\uparrow},\ensuremath{\downarrow}$, of the two leads solely in the FM configuration. The bias behavior of the NEIEC components can be selectively tuned with the BF of the free and pinned FM layers, thus opening a new avenue for controlling experimentally ${T}_{\ensuremath{\perp}}$.

Simulation of electrocatalytic hydrogen production by a bioinspired catalyst anchored to a pyrite electrode.

Abstract: The possibility of using the active site, the [FeFe]H cluster, of the bacterial di-iron hydrogenases as a catalyst for hydrogen production from water by electro- or photocatalysis is of current scientific and technological interest We present here a theoretical study of hydrogen production by a modified [FeFe]H cluster stably linked to a pyrite electrode immersed in acidified water We employed state-of-the-art electronic-structure and first-principles molecular-dynamics methods We found that a stable sulfur link of the cluster to the surface analogous to that linking the cluster to its enzyme environment cannot be made However, we have discovered a modification of the cluster which does form a stable, tridentate link to the surface The pyrite electrode readily produces hydrogen from acidified water when functionalized with the modified cluster, which remains stable throughout the hydrogen production cycle

Theoretical Design by First Principles Molecular Dynamics of a Bioinspired Electrode−Catalyst System for Electrocatalytic Hydrogen Production from Acidified Water

Abstract: Bacterial di-iron hydrogenases produce hydrogen efficiently from water. Accordingly, we have studied by first-principles molecular-dynamics simulations (FPMD) electrocatalytic hydrogen production from acidified water by their common active site, the [FeFe]H cluster, extracted from the enzyme and linked directly to the (100) surface of a pyrite electrode. We found that the cluster could not be attached stably to the surface via a thiol link analogous to that which attaches it to the rest of the enzyme, despite the similarity of the (100) pyrite surface to the Fe4S4 cubane to which it is linked in the enzyme. We report here a systematic sequence of modifications of the structure and composition of the cluster devised to maintain the structural stability of the pyrite/cluster complex in water throughout its hydrogen production cycle, an example of the molecular design of a complex system by FPMD.

Field-like spin torque in magnetic tunnel junctions

Abstract: We show that the exchange splitting asymmetry between the left and right ferromagnetic leads in non-collinear magnetic tunnel junctions (MTJ) tunes the bias behavior of the field-like spin torque, T⊥. These results can be understood by our recently derived general expression, which relates the non-collinear T⊥ to the algebraic sum of four independent non-equilibrium interlayer exchange couplings (IEC) solely in collinear configurations.

A model approach to modelling

Abstract: When Roberto Car and Michele Parrinello started collaborating in the early 1980s, electronic structure calculations and molecular dynamics (MD) simulations of atomic configurations were virtually isolated from each other. But Car and Parrinello combined their individual expertise to create a method that bridged the gap between the two communities and changed the approach to materials modelling in a radical way1. The Car–Parrinello ab initio molecular dynamics method was conceived and developed within the theoretical physics community in Trieste, in a period in which SISSA (Scuola Internazionale Superiore di Studi Avanzati) was still a very young institute and was practically overlapping with the well-established International Centre of Theoretical Physics (ICTP). The presence of many bright physicists and the continuous flow of international visitors made the atmosphere highly stimulating. Discussions went on virtually without interruption, even during the occasional swims around the shores of the Miramare Castle (pictured), only a few steps away from the ICTP building. Car’s background in density functional theory (DFT) and extreme enthusiasm was perfectly complemented by Parrinello’s knowledge of statistical mechanics and molecular dynamics, in addition to his reflective attitude, resulting in an ideal combination of personality and scientific expertise. The paper reporting the Car–Parrinello method1 was published in November 1985, and on the occasion of its twenty-fifth anniversary we explore the origin of the work and its long-lasting impact. What emerges from the Interviews with the two scientists2,3 is that the main motivation for their efforts were the limitations suffered by both DFT and classical MD, which effectively restricted their application to the realistic simulation of condensed matter at finite temperature to specific cases. Pure DFT was mainly applicable to the electronic structures of ordered and homogenous systems. On the other hand, the forces between atoms used in MD did not take into account the fact that the electronic potentials varied with the atomic movement during the progress of a simulation. By using DFT to calculate the potential ‘felt’ by atoms and letting such potential evolve with each step of the simulation, the Car–Parrinello method allowed a much wider range of disordered and therefore more realistic materials systems to be studied. This was beautifully demonstrated by applying the method to amorphous silicon, which led to results on both the atomic structure and the electronic properties that were in very good agreement with experimental observations4. More generally, the ability to follow the evolution of the electronic potential has allowed important studies of chemical reactions occurring, for example, in liquids and large biomolecules. In his Commentary5, besides recalling the fundamental concepts behind the Car–Parrinello method, Jürgen Hafner describes the highly inspirational role that the approach had in stimulating the creation of a series of first-principles computational codes that are still used widely today. A special role for these codes is their application to phenomena that are difficult to study experimentally, such as those occurring at high pressure or during complex and fast chemical reactions. An aspect of these codes that is not always appreciated is their importance in industrial applications. The CASTEP code6, conceived and developed by Mike Payne and collaborators at the University of Cambridge, is a perfect example. Since its commercialization, it has been the highest source of revenue for the university in the physical sciences. The code is the fundamental part of the software package ‘Materials Studio’, developed and commercialized by Accelrys, with the aim of providing a powerful tool to generic users with no specific knowledge of code writing. According to Gerhard Goldbeck-Wood, director of product marketing for Materials Studio, the software package is used by chemical companies that work in fields such as catalysis or high-k dielectrics, to name just a few. Most importantly an essential aim of these types of code is their use in feasibility tests, which has substantial economical benefits. Unlike the discovery of a new molecule or the observation of a new phenomenon, it is difficult to appreciate the success that a computational technique is likely to have when it is first introduced. After a quarter of a century however, it is clear that the Car–Parrinello method has been ground-breaking within the field of computational materials science and has had an enormous impact on fundamental science and applications in a wide range of fields, from solid-state materials physics to chemistry and biology. Still, the approach was originally conceived by two scientists motivated solely by their passion for science and their desire to understand nature, with no specific agenda with regard to the possible commercial value of their work. This should send a strong message to decision makers in charge of funding research. ❐

From ab initio onwards

Abstract: Roberto Car tells Nature Materials how the Car–Parrinello molecular dynamics method originated and how his research career has evolved since then.

The isotope-effect in the phase transition of KDP: New insights from ab initio path-integral simulations

Abstract: We investigate the quantum-mechanical localization of protonated and deterated isotopes in the symmetric low-barrier hydrogen-bonds of potassium dihydrogen phosphate (KDP) crystals in the paraelectric phase. The spatial density distributions of these hydrogen atoms are suspected to be responsible for the surprisingly large isotope effect observed for the ferroelectric phase transition in KDP. We employ ab initio path integral molecular dynamics simulations to obtain the nuclear real-space and momentum-space densities n(R) and n(k) of protons and deuterons, which are compared to experimental Neutron Compton Scattering data. Our results suggest a qualitative difference in the nature of the paraelectic phase in KDP between the two isotopes. We are able to discriminate between real quantum delocalization and vibration-assisted hopping and thus provide evidence for two distinct mechanisms of the ferroelectric phase transition in this class of materials.