Showing papers by "Roberto Car published in 1994"

Acceleration schemes for ab initio molecular-dynamics simulations and electronic-structure calculations.

Abstract: We study the convergence and the stability of fictitious dynamical methods for electrons. first, we show that a particular damped second-order dynamics has a much faster rate of convergence to the ground state than first-order steepest-descent algorithms while retaining their numerical cost per time step. Our damped dynamics has efficiency comparable to that of conjugate gradient methods in typical electronic minimization problems. Then, we analyze the factors that limit the size of the integration time step in approaches based on plane-wave expansions. The maximum allowed time step is dictated by the highest frequency components of the fictitious electronic dynamics. These can result either from the larger wave vector components of the kinetic energy or from the small wave vector components of the Coulomb potential giving rise to the so called charge sloshing problem. We show how to eliminate large wave vector instabilities by adopting a preconditioning scheme in the context of Car-Parrinello ab initio molecular-dynamics simulations of the ionic motion. We also show how to solve the charge sloshing problem when this is present. We substantiate our theoretical analysis with numerical tests on a number of different silicon and carbon systems having both insulating and metallic character.

Ab initio study of positron trapping at a vacancy in GaAs.

Abstract: We present a first-principles study of positron trapping at a negatively charged As vacancy in GaAs Lattice relaxations induced both by the presence of the defect and of the positron have been included in a self-consistent way In the presence of a positron, the volume of the vacancy increases and its symmetry is lowered The positron wave function is well localized in the defect Calculated positron lifetime and angular correlations of annihilation photons are in good agreement with recent experiments

Ab initio molecular dynamics with a finite-temperature density functional

Abstract: The unified method for molecular dynamics and density functional theory (MD/DF) introduced by Car and Parrinello is based on zero-temperature density functional theory. We have incorporated the finite-temperature extension of density functional theory proposed by Mermin into a consistent fictitious Lagrangian framework. Such an extension of the original MD/DF method is desirable for two rather different reasons. First this framework provides a general method to treat electronic states at finite temperature or in non-equilibrium excited states. Second it can alleviate certain practical problems that arise when Kohn-Sham DF methods in general and MD/DF in particular are applied to metallic and near-metallic systems. Our approach involves dynamically varying occupation numbers which is important for states near the Fermi energy. We show that the added degrees of freedom of these states can be used to accelerate the convergence to the electronic ground state. In MD simulations this improved response of the electrons also leads to an increase in the rate of energy transfer from the ionic to the electronic degrees of freedom. Our method is illustrated by calculations on crystalline metallic carbon and simulations of liquid silicon.

Aluminum on Si(100): Growth and structure of the first layer

Abstract: The growth and the (electronic) structure of a single layer of aluminum on the Si(100) surface are examined by means of first principles calculations. The calculated local density of states is used to interprete scanning tunneling microscopy data. We identify the adsorption sites that give rise to the polymerization‐like one‐dimensional growth of Al. The Al‐induced surface states of the p(2×2) covered Si(100) surface are calculated. The surface bandgap is increased, as compared to the clean Si(100) surface, which means that the first adsorbed Al layer passivates the surface electrically.

Low- and high-temperature phases of a Pb monolayer on Ge(111) from ab initio molecular dynamics

Abstract: We present an ab initio molecular dynamics study of the structure and dynamics of a close-packed monolayer of Pb on the Ge(111) surface, with coverage FTHETA=4/3, at different temperatures. The room-temperature (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ) structure is characterized by large in-plane fluctuations of the Pb adatoms, and agrees well with recent x-ray standing wave data. At T\ensuremath{\sim}800 K we observe a (1\ifmmode\times\else\texttimes\fi{}1) disordered structure showing in-plane diffusion of the Pb atoms. Disordering is confined to the plane of the overlayer. A strong correlation with the solid Ge substrate is present, leading to preferential residence sites and diffusion paths of the Pb atoms on the surface. The calculated local density of states for the high-temperature phase is found to agree with recent scanning tunnel microscope observations, which show a simply Pb-terminated (1\ifmmode\times\else\texttimes\fi{}1) surface with an apparent coverage FTHETA=1.