265 research outputs found

    Automation methodologies and large-scale validation for GWGW, towards high-throughput GWGW calculations

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    The search for new materials, based on computational screening, relies on methods that accurately predict, in an automatic manner, total energy, atomic-scale geometries, and other fundamental characteristics of materials. Many technologically important material properties directly stem from the electronic structure of a material, but the usual workhorse for total energies, namely density-functional theory, is plagued by fundamental shortcomings and errors from approximate exchange-correlation functionals in its prediction of the electronic structure. At variance, the GWGW method is currently the state-of-the-art {\em ab initio} approach for accurate electronic structure. It is mostly used to perturbatively correct density-functional theory results, but is however computationally demanding and also requires expert knowledge to give accurate results. Accordingly, it is not presently used in high-throughput screening: fully automatized algorithms for setting up the calculations and determining convergence are lacking. In this work we develop such a method and, as a first application, use it to validate the accuracy of G0W0G_0W_0 using the PBE starting point, and the Godby-Needs plasmon pole model (G0W0GNG_0W_0^\textrm{GN}@PBE), on a set of about 80 solids. The results of the automatic convergence study utilized provides valuable insights. Indeed, we find correlations between computational parameters that can be used to further improve the automatization of GWGW calculations. Moreover, we find that G0W0GNG_0W_0^\textrm{GN}@PBE shows a correlation between the PBE and the G0W0GNG_0W_0^\textrm{GN}@PBE gaps that is much stronger than that between GWGW and experimental gaps. However, the G0W0GNG_0W_0^\textrm{GN}@PBE gaps still describe the experimental gaps more accurately than a linear model based on the PBE gaps.Comment: 12 pages, 11 figure

    Room temperature Peierls distortion in small radius nanotubes

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    By means of {\it ab initio} simulations, we investigate the phonon band structure and electron-phonon coupling in small 4-\AA diameter nanotubes. We show that both the C(5,0) and C(3,3) tubes undergo above room temperature a Peierls transition mediated by an acoustical long-wavelength and an optical q=2kFq=2k_F phonons respectively. In the armchair geometry, we verify that the electron-phonon coupling parameter λ\lambda originates mainly from phonons at q=2kFq=2k_F and is strongly enhanced when the diameter decreases. These results question the origin of superconductivity in small diameter nanotubes.Comment: submitted 21oct2004 accepted 6jan2005 (Phys.Rev.Lett.

    Band Offsets at the Si/SiO2_2 Interface from Many-Body Perturbation Theory

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    We use many-body perturbation theory, the state-of-the-art method for band gap calculations, to compute the band offsets at the Si/SiO2_2 interface. We examine the adequacy of the usual approximations in this context. We show that (i) the separate treatment of band-structure and potential lineup contributions, the latter being evaluated within density-functional theory, is justified, (ii) most plasmon-pole models lead to inaccuracies in the absolute quasiparticle corrections, (iii) vertex corrections can be neglected, (iv) eigenenergy self-consistency is adequate. Our theoretical offsets agree with the experimental ones within 0.3 eV

    Origin of magnetism and quasiparticles properties in Cr-doped TiO2_2

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    Combining LSDA+UU and an analysis of superexchange interactions beyond DFT, we describe the magnetic ground states in rutile and anatase Cr-doped TiO2_2. In parallel, we correct our LSDA+UU ground state through GW corrections (GWGW@LSDA+UU) that reproduce the position of impurity states and the band gaps in satisfying agreement with experiments. Because of the different topological coordinations of Cr-Cr bonds in the ground states of rutile and anatase, superexchange interactions induce either ferromagnetic or antiferromagnetic couplings of Cr ions. In Cr-doped anatase, this interaction leads to a new mechanism which stabilizes a ferromagnetic ground state, in keeping with experimental evidence, without the need to invoke F-center exchange.Comment: 5<pages, 4 figure

    The PseudoDojo: Training and grading a 85 element optimized norm-conserving pseudopotential table

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    First-principles calculations in crystalline structures are often performed with a planewave basis set. To make the number of basis functions tractable two approximations are usually introduced: core electrons are frozen and the diverging Coulomb potential near the nucleus is replaced by a smoother expression. The norm-conserving pseudopotential was the first successful method to apply these approximations in a fully ab initio way. Later on, more efficient and more exact approaches were developed based on the ultrasoft and the projector augmented wave formalisms. These formalisms are however more complex and developing new features in these frameworks is usually more difficult than in the norm-conserving framework. Most of the existing tables of norm- conserving pseudopotentials, generated long ago, do not include the latest developments, are not systematically tested or are not designed primarily for high accuracy. In this paper, we present our PseudoDojo framework for developing and testing full tables of pseudopotentials, and demonstrate it with a new table generated with the ONCVPSP approach. The PseudoDojo is an open source project, building on the AbiPy package, for developing and systematically testing pseudopotentials. At present it contains 7 different batteries of tests executed with ABINIT, which are performed as a function of the energy cutoff. The results of these tests are then used to provide hints for the energy cutoff for actual production calculations. Our final set contains 141 pseudopotentials split into a standard and a stringent accuracy table. In total around 70.000 calculations were performed to test the pseudopotentials. The process of developing the final table led to new insights into the effects of both the core-valence partitioning and the non-linear core corrections on the stability, convergence, and transferability of norm-conserving pseudopotentials. ...Comment: abstract truncated, 17 pages, 25 figures, 8 table

    Epitaxially strained [001]-(PbTiO3_3)1_1(PbZrO3_3)1_1 superlattice and PbTiO3_3 from first principles

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    The effect of layer-by-layer heterostructuring and epitaxial strain on lattice instabilities and related ferroelectric properties is investigated from first principles for the [001]-(PbTiO3_3)1_1(PbZrO3_3)1_1 superlattice and pure PbTiO3_3 on a cubic substrate. The results for the superlattice show an enhancement of the stability of the monoclinic r-phase with respect to pure PbTiO3_3. Analysis of the lattice instabilities of the relaxed centrosymmetric reference structure computed within density functional perturbation theory suggests that this results from the presence of two unstable zone-center modes, one confined in the PbTiO3_3 layer and one in the PbZrO3_3 layer, which produce in-plane and normal components of the polarization, respectively. The zero-temperature dielectric response is computed and shown to be enhanced not only near the phase boundaries, but throughout the r-phase. Analysis of the analogous calculation for pure PbTiO3_3 is consistent with this interpretation, and suggests useful approaches to engineering the dielectric properties of artificially structured perovskite oxides.Comment: 8 pages, 5 figure

    The bandstructure of gold from many-body perturbation theory

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    The bandstructure of gold is calculated using many-body perturbation theory (MBPT). Different approximations within the GW approach are considered. Standard single shot G0W0 corrections shift the unoccupied bands up by ~0.2 eV and the first sp-like occupied band down by ~0.4 eV, while leaving unchanged the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the wavefunctions lowers the occupied 5d bands by 0.35 eV. Globally, many-body effects achieve an opening of the interband gap (5d-6sp gap) of 0.35 to 0.75 eV approaching the experimental results. Finally, the quasiparticle bandstructure is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and Ernzerhof) hybrid functional
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