Search results for "ELECTRONIC STRUCTURE"
showing 10 items of 722 documents
Diagrammatic expansion for positive spectral functions beyond GW : Application to vertex corrections in the electron gas
2014
We present a diagrammatic approach to construct self-energy approximations within many-body perturbation theory with positive spectral properties. The method cures the problem of negative spectral functions which arises from a straightforward inclusion of vertex diagrams beyond the GW approximation. Our approach consists of a two-steps procedure: we first express the approximate many-body self-energy as a product of half-diagrams and then identify the minimal number of half-diagrams to add in order to form a perfect square. The resulting self-energy is an unconventional sum of self-energy diagrams in which the internal lines of half a diagram are time-ordered Green's functions whereas those…
The Dalton quantum chemistry program system
2013
Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree-Fock, Kohn-Sham, multiconfigurational self-consistent-field, MOller-Plesset, confi ...
Modelling of defects and surfaces in perovskite ferroelectrics
2003
The results of electronic structure calculations for different terminations of SrTiO 3 (100) and (110) perovskite thin films are discussed. These calculations are based on the ab initio Hartree-Fock (HF) method and Density Functional Theory (DFT). Results are compared with previous ab initio plane-wave LDA and classical Shell Model (SM) calculations. Calculated considerable increase of the Ti-O chemical bond covalency nearby the surface is confirmed by experimental data. Our quantum chemical calculations performed by means of the intermediate neglect of differential overlap (INDO) method confirm the existence of self-trapped electrons in KNbO 3 , KTaO 3 and BaTiO 3 crystals. The relevant la…
Quantum chemical modelling of point defects in KNbO3 perovskite crystals
2000
Abstract We present results of semi-empirical quantum chemical calculations for several perovskite KNb x Ta 1−x O 3 (KTN) solid solutions, as well as point intrinsic defects – F centers and hole polarons bound to K vacancy – in KNbO 3 . Method of the intermediate neglect of the differential overlap (INDO) was combined with typically 320-atom supercells and atomic geometry optimization. Analysis of the optimized atomic and electronic structure has clearly demonstrated that several nearest Nb atoms substituting for Ta in KTaO 3 – unlike Ta impurities in KNbO 3 – reveal the self-ordering effect, which probably triggers the ferroelectricity observed in KTN. We predict co-existence of one-site (…
Exploration of the Potential Energy Landscape of Aniline Using CASSCF and XMCQDPT2 Electronic Structure Calculations
2016
In this chapter, the tools introduced in Chap. 2 are applied to the study of the photochemistry of aniline. This work has been initiated during a six-month fellowship in the group of Professor Helen Fielding in the Chemistry Department of University College London, in the framework of the FASTQUAST Initial Training Network.
Disentangling size effects and spectral inhomogeneity in carbon nanodots by ultrafast dynamical hole-burning.
2018
Carbon nanodots (CDs) are a novel family of nanomaterials exhibiting unique optical properties. In particular, their bright and tunable fluorescence redefines the paradigm of carbon as a "black" material and is considered very appealing for many applications. While the field keeps growing, understanding CDs fundamental properties and relating them to their variable structures becomes more and more critical. Two crucial problems concern the effect of size on the electronic structure of CDs, and to what extent their optical properties are influenced by structural disorder. Furthermore, it remains largely unclear whether traditional concepts borrowed from the photo-physics of semiconductor qua…
Proton, Hydroxide Ion, and Oxide Ion Affinities of Closed-Shell Oxides: Importance for the Hydration Reaction and Correlation to Electronic Structure
2019
Phenomenologically, the enthalpy of the dissociative water incorporation (hydration) of oxides is often found to be more favorable for more basic oxides. In the present work, we investigate proton,...
Photomagnetism of Molecular Systems
2001
This article reviews various classes of transition metal compounds which display in the solid state light sensitive electronic structure modifications accompanied by drastic changes of their magnetic and/or optical properties, i.e., photomagnetism. Selected examples from this rapidly expanding field, include spin crossover, stilbenoid and nitrosyl complexes, as well as Prussian blue analogues.
Theoretical study of the stabilization of cubic-phaseZrO2by impurities
1994
We have performed a thermodynamical analysis of the phase diagrams for ${\mathrm{ZrO}}_{2}$-CaO and ${\mathrm{ZrO}}_{2}$-MgO solid solutions which has demonstrated that differential heats of mixing are important parameters determining the stabilization of the cubic phase of ${\mathrm{ZrO}}_{2}$ by impurities. It is shown that the differential heats of mixing in the cubic phase of these systems should be lower than in the tetragonal phase. To understand this effect we have studied the electronic and geometrical structures of the pure and doped ${\mathrm{ZrO}}_{2}$ crystals. Three computational techniques were employed: the ab initio Hartree-Fock pseudopotential method is used to study the at…
Impact of the Substituents on the Electronic Structure of the Four Most Stable Tautomers of Purine and Their Adenine Analogues
2020
Substituent effects at the C2-, C8-, and N-positions of adenine and purine on the structural and π-electronic changes in their four tautomers were studied using the B97D3/aug-cc-pvdz computational level. The effect of various substituents (NO2, CN, CHO, Cl, F, H, Me, OMe, OH, and NH2) was characterized by the charge of the substituent active region (cSAR) approach and Hammett substituent constants σ. It has been found that for both adenine and purine derivatives, substituents from the C8–X position have a stronger influence on their electronic structure than from the C2–X and N–X positions. The presence of the amino group in adenine enhances the substituent effect compared to that which occ…