1,721,010 research outputs found
Ab initio pseudopotential calculation of the equilibrium structure of tin monoxide
No full textWe present an ab initio pseudopotential calculation of the structural properties of stannous oxide SnO. We discuss the delicate balance of different contributions to the cohesion of this material, and compare the monoxide to the dioxide SnO2. We point out how different choices concerning the pseudopotential of tin may dramatically influence the resulting equilibrium structure of SnO, and show that the physically most appropriate choice leads to excellent agreement with experiment
An efficient method for calculating quasiparticle energies in semiconductors
No full textWe present a method for the efficient calculation of the electronic structure of semiconductors within the GW approach. It approximately includes dynamical-screening and local-field effects, previously disregarded in simplified GW approaches, without increasing the computational effort. Such effects substantially affect the gap corrections. We find quasiparticle shifts in good agreement with the complete GW calculations or experiment for Si, AlAs, GaAs and ZnSe
Model dielectric function for semiconductors
No full textWe present a model for the dielectric function of semiconductors. It has been tested successfully for
Si, Ge, GaAs, and ZnSe. In conjunction with the single plasmon-pole approximation it yields plasmonenergy
dispersions in fair agreement with experiments. It allows one, moreover, to deduce an analytical
expression for the Coulomb-hole part of the static self-energy operator
An efficient method for calculating quasiparticle energies in semiconductors
No full textWe present a method for the efficient calculation of the electronic structure of semiconductors within the GW approach. It approximately includes dynamical-screening and local-field effects, previously disregarded in simplified GW approaches, without increasing the computational effort. Such effects substantially affect the gap corrections. We find quasiparticle shifts in good agreement with the complete GW calculations or experiment for Si, AlAs, GaAs and ZnSe
Many-body effects in the electronic spectra of cubic BN
No full textWe present state of the art first-principles calculations of optical spectra and the loss function of bulk cubic
boron nitride sc-BNd, starting from a density functional Kohn-Sham band structure. We investigate the influence
of many-body effects beyond the random phase approximation (RPA) on the optical spectra through the
inclusion of self-energy and excitonic effects by a GW calculation and the solution of the Bethe-Salpeter
equation. For the loss function we only perform RPA calculations, since Bethe-Salpeter results are already
available in the literature. We show to which extent, and in which kind of spectra, the description of manybody
effects is important for a meaningful comparison with experiment, and when they can be neglected due
to mutual cancellation. We also present results obtained within time-dependent density functional theory, both
in the adiabatic local density approximation (TDLDA) and using a recently proposed long-range approximation
for the exchange-correlation kernel. Our results show that the latter corrects a big part of the error with
respect to RPA or TDLDA; however, the corrections are not sufficient to qualify the method for further
quantitative predictions, in particular for the study of the optical gap. In fact, since experiments often quote a
relatively low (around 6.4 eV) band gap, whereas the calculated optical absorption spectrum already in the
random-phase approximation appears blueshifted by more than 2 eV with respect to the available experimental
curve, we study in particular the question of the optical gap in this material. It turns out that, although there is
evidence for a weakly bound exciton in c-BN, the optical gap of pure monocrystalline cubic BN should be
around 11 eV, hence significantly bigger than has sometimes been quoted from experiments
Ab-initio calculations for the electronic spectra of cubic and hexagonal BN
No full textWe present state of the art fist-principles calculations for the optical spectra and the loss functions of bulk boron nitride in the cubic (c-BN) and in the hexagonal (h-BN) phases. We start from a DFT-LDA density functional Khon-Sham bandstructure to investigate the influence of many-body effects beyond the Random Phase Approximation (RPA) on the optical spectra through the inclusion of self-energy and excitonic effects by a GW calculation and the solution of the Bethe-Salpeter equation. For the loss function we only perform RPA calculations. We show to which extent the description of many-body effects is important for a meaningiful comparison with experiment, and when they can be neglected
Quasiparticle energies and band gaps in semiconductors determined with an efficient DFT-GW scheme
No full textQuasiparticle energies and band gaps in semiconductors determined with an efficient DFT-GW schem
Local fields and dynamical screening effects on the semiconductors band gap
No full textLocal fields and dynamical screening effects
on the semiconductors band ga
First Principles simulations
No full textIn this paper we outline the major features of the “ab-initio” simulation scheme of Car and
Parrinello, focusing on the physical ideas and computational details at the basis of its efficiency
and success. We briefly review the main applications of the method. We discuss the limitations
of the standard scheme, as well as recent developments proposed in order to extend the reach
of the method. Moreover, we consider more in detail two specific subjects. First, we describe a simple improvement
(Gradient Corrections) on the basic approximation of the "ab-initio" simulation, ie the
Local Density Approximation. These corrections can be easily and efficiently included in the
Car-Parrinello code, bringing computed structural and cohesive properties significantly closer
to their experimental values. Finally, we discuss the choice of the pseudopotential, with special
attention to the possibilities and limitations of the last generation of soft pseudopotentials
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