138 research outputs found
SaX: An open source package for electronic-structure and optical-properties calculations in the GW approximation
No full textWe present here SaX (Self-energies and eXcitations), a plane-waves package aimed at electronic-structure and optical-properties calculations in the GW framework, namely using the GW approximation for quasiparticle properties and the Bethe-Salpeter equation for the excitonic effects. The code is mostly written in FORTRAN90 in a modern style, with extensive use of data abstraction (i.e. objects). Sax employs state of the art techniques and can treat large systems. The package is released with an open source license and can be also download from http://www.sax-project.org/
QMMMW: A wrapper for QM/MM simulations with QUANTUM ESPRESSO and LAMMPS
No full textWe present QMMMW, a new program aimed at performing Quantum Mechanics/Molecular Mechanics (QM/MM) molecular dynamics. The package operates as a wrapper that patches PWscf code included in the QUANTUM ESPRESSO distribution and LAMMPS Molecular Dynamics Simulator. It is designed with a paradigm based on three guidelines: (i) minimal amount of modifications on the parent codes, (ii) flexibility and computational efficiency of the communication layer and (iii) accuracy of the Hamiltonian describing the interaction between the QM and MM subsystems. These three features are seldom present simultaneously in other implementations of QMMM. The QMMMW project is hosted by qe-forge at (http://qe-forge.org/gf/project/qmmmw/)
Ab initio complex band structure of conjugated polymers: Effects of hydrid density functional theory and GW schemes
No full text09.08.12 meb Author version. Publisher allows this to be adde
Unraveling effects of disorder on the electronic structure of SiO(2) from first principles
No full textWe present a first-principles systematic study of the electronic structure of SiO(2) including the crystalline polymorphs alpha quartz and beta cristobalite, and different types of disorder leading to the amorphous phase. We start from calculations within density functional theory and proceed to more sophisticated quasiparticle calculations according to the GW scheme. Our results show that different origins of disorder have also different impact on atomic and electronic-density fluctuations, which affect the electronic structure and, in particular, the size of the mobility gap in each case.RTN EU[HPRN-CT-2002-00317]FIRB-MIUR Italy-Canada[RBIN06JB4C]FAPESPCNP
SiO(2) in density functional theory and beyond
No full textWe present the first-principle electronic structure calculation on an amorphous material including many-body corrections within the GW approximation. We show that the inclusion of the local field effects in the exchange-correlation potential is crucial to quantitatively describe amorphous systems and defect states. We show that the mobility gap of amorphous silica coincides with the band gap of quartz, contrary to the traditional picture and the densityfunctional theory results. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Silicon nanocrystallites in a SiO2 matrix: Role of disorder and size
No full textWe compare, through first-principles pseudopotential calculations, the structural, electronic, and optical properties of different size silicon nanoclusters embedded in a SiO2 crystalline or amorphous matrix with that of freestanding, hydrogenated, and hydroxided silicon nanoclusters of corresponding size and shape. We find that the largest effect on the optoelectronic behavior is due to the amorphization of the embedded nanocluster. In that, the amorphization reduces the fundamental gap while increasing the absorption strength in the visible range. Increasing the nanocluster size does not change substantially this picture but only leads to the reduction in the absorption threshold, following the quantum confinement rule. Finally, through the calculation of the optical absorption spectra both in an independent-particle and a many-body approach, we show that the effect of local fields is crucial for describing properly the optical behavior of the crystalline case while it is of minor importance for amorphous systems
Optical properties of silicon nanocrystallites in SiO2 matrix: Crystalline vs. amorphous case
No full textWithin a first-principles framework we show the dependence of the optical properties of silicon nanocrystallites embedded in a silica matrix on the crystalline vs. amorphous order of the system. Moreover we calculate how many-body effects modify the electronic and optical properties of the embedded silicon nanodots. A discussion about the different roles played by dimensionality, interface properties, disorder and excitonic effects on the electronic and optical properties of the confined nanostructures is presented. (C) 2008 Elsevier Ltd. All rights reserved
Evidence of enhanced photocurrent response in corannulene films
No full textExperimental optical absorption and photoconductivity spectra of thin films with GW–BSE theoretical predictions provide evidence for diffuse super atomic molecular orbitals (SAMOs) in corannulene, C20H10.</p
First-principles characterization of Mg low-index surfaces : Structure, reconstructions, and surface core-level shifts
No full textIn this paper, first-principles calculations provide structural characterization of three low-index Mg surfaces - Mg(0001), Mg(1010), and Mg(1120) - and their respective surface core-level shifts (SCLSs). Inspired by the close similarities between Be and Mg surfaces, we also explore the reconstruction of Mg(1120). Through the calculation of surface energies and the use of the angular-component decomposed density of states, we show that reconstructions are likely to occur at the Mg(1120) surface, similarly to what was found earlier for Be(1120). Indeed, the surface energy of some of the explored reconstructions is slightly lower than that of the unreconstructed surface. In addition, because of lattice symmetry, the morphology of the unreconstructed surface (1120) results in a steplike zig-zag chain packing, with topmost chains supporting a resonant, quasi-one-dimensional (1D), partially filled electronic state. As the presence of partially filled quasi-1D bands is a necessary condition for Peierls-like dimerization, we verify that the undimerized surface chain remains stable with respect to it. Some of the reconstructions, namely, the 2×1 and 3×1 added row reconstructions, induce a stronger relaxation of the topmost chains, increasing the coupling with lower layers and thus significantly damping the quasi-1D character of this state. The original approach followed offers a common and general framework to identify quasi-1D bands - even in the case of resonant electronic surface states - and to meaningfully compare calculated and measured SCLSs even in the presence of multicomponent peak contributions
Buckybowl superatom states: a unique route for electron transport?
No full textA unique paradigm for intermolecular charge transport mediated by diffuse atomic-like orbital (SAMOs), typically present in conjugated hollow shaped molecules, is investigated for C20H10 molecular fragments by means of G0W0 theory. Inclusion of many body screening and polarization effects is seen to be important for accurate prediction of electronic properties involving these diffuse orbitals. Theoretical predictions are made for the series of bowl-shaped fullerene fragments, C20H10, C30H10, C40H10, C50H10. Interesting results are found for the LUMO-SAMO energy gap in C20H10, which is shown to be nearly an order of magnitude lower that that determined for C60. Given the ability to support bowl fragments on metal surfaces, these results suggest the concrete possibility for exploiting SAMO-mediated electron transport in supramolecular conducting layers
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