2,672 research outputs found
First-principle molecular dynamics with ultrasoft pseudopotentials: parallel implementation and application to extended bio-inorganic system
We present a plane-wave ultrasoft pseudopotential implementation of
first-principle molecular dynamics, which is well suited to model large
molecular systems containing transition metal centers. We describe an efficient
strategy for parallelization that includes special features to deal with the
augmented charge in the contest of Vanderbilt's ultrasoft pseudopotentials. We
also discuss a simple approach to model molecular systems with a net charge
and/or large dipole/quadrupole moments. We present test applications to
manganese and iron porphyrins representative of a large class of biologically
relevant metallorganic systems. Our results show that accurate
Density-Functional Theory calculations on systems with several hundred atoms
are feasible with access to moderate computational resources.Comment: 29 pages, 4 Postscript figures, revtex
DC Conductance of Molecular Wires
Inspired by the work of Kamenev and Kohn, we present a general discussion of
the two-terminal dc conductance of molecular devices within the framework of
Time Dependent Current-Density Functional Theory. We derive a formally exact
expression for the adiabatic conductance and we discuss the dynamical
corrections. For junctions made of long molecular chains that can be either
metallic or insulating, we derive the exact asymptotic behavior of the
adiabatic conductance as a function of the chain's length. Our results follow
from the analytic structure of the bands of a periodic molecular chain and a
compact expression for the Green's functions. In the case of an insulating
chain, not only do we obtain the exponentially decaying factors, but also the
corresponding amplitudes, which depend very sensitively on the electronic
properties of the contacts. We illustrate the theory by a numerical study of a
simple insulating structure connected to two metallic jellium leads.Comment: 15 pgs and 9 figure
First principles study of adsorbed Cu_n (n=1-4) microclusters on MgO(100): structural and electronic properties
We present a density functional study of the structural and electronic
properties of small Cu_n (n=1,4) aggregates on defect-free MgO(100). The
calculations employ a slab geometry with periodic boundary conditions,
supercells with up to 76 atoms, and include full relaxation of the surface
layer and of all adsorbed atoms. The preferred adsorption site for a single Cu
adatom is on top of an oxygen atom. The adsorption energy and Cu-O distance are
E_S-A = 0.99 eV and d_S-A = 2.04 Angstroems using the Perdew-Wang gradient
corrected exchange correlation functional. The saddle point for surface
diffusion is at the "hollow" site, with a diffusion barrier of around 0.45 eV.
For the adsorbed copper dimer, two geometries, one parallel and one
perpendicular to the surface, are very close in energy. For the adsorbed Cu_3,
a linear configuration is preferred to the triangular geometry. As for the
tetramer, the most stable adsorbed geometry for Cu_4 is a rhombus. The
adsorption energy per Cu atom decreases with increasing the size of the
cluster, while the Cu-Cu cohesive energy increases, rapidly becoming more
important than the adsorption energy.Comment: Major revision, Latex(2e) document, 23 pages, 11 figures, accepted
for publication in J. of Chem. Phys., paper available at
http://irrmawww.epfl.ch/vm/vm_wor
Electronic Properties of Molecules and Surfaces with a Self\uad-Consistent Interatomic van der Waals Density Functional.
How strong is the effect of van der Waals (vdW) interactions on the electronic properties of molecules
and extended systems? To answer this question, we derived a fully self-consistent implementation of the
density-dependent interatomic vdW functional of Tkatchenko and Scheffler [Phys. Rev. Lett. 102, 073005
(2009)]. Not surprisingly, vdW self-consistency leads to tiny modifications of the structure, stability, and
electronic properties of molecular dimers and crystals. However, unexpectedly large effects were found in
the binding energies, distances, and electrostatic moments of highly polarizable alkali-metal dimers. Most
importantly, vdW interactions induced complex and sizable electronic charge redistribution in the vicinity
of metallic surfaces and at organic-metal interfaces. As a result, a substantial influence on the computed
work functions was found, revealing a nontrivial connection between electrostatics and long-range electron
correlation effects
A Branching Time Model of CSP
I present a branching time model of CSP that is finer than all other models
of CSP proposed thus far. It is obtained by taking a semantic equivalence from
the linear time - branching time spectrum, namely divergence-preserving coupled
similarity, and showing that it is a congruence for the operators of CSP. This
equivalence belongs to the bisimulation family of semantic equivalences, in the
sense that on transition systems without internal actions it coincides with
strong bisimilarity. Nevertheless, enough of the equational laws of CSP remain
to obtain a complete axiomatisation for closed, recursion-free terms.Comment: Dedicated to Bill Roscoe, on the occasion of his 60th birthda
Acceleration Schemes for Ab-Initio Molecular Dynamics and Electronic Structure Calculations
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 analyse 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 large wavevector components of the kinetic energy or from the
small wavevector components of the Coulomb potential giving rise to the so
called {\it charge sloshing} problem. We show how to eliminate large wavevector
instabilities by adopting a preconditioning scheme that is implemented here for
the first-time 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.Comment: RevTex, 9 figures available upon request, to appear in Phys. Rev.
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