Electronic structure ground state properties

In this contribution we have reviewed the applicability, accuracy and computational efficiency of the local spin density functional approach to the chemistry of transition metal complexes and clusters using a linear combination of Gaussian-type orbital basis set for the calculation of electronic structures, ground state geometries and vibrational properties. 

Molecular Properties and Spectra 3.14.3.1 Electronic Structure Ground State... 

A series of aromatic nitrenes substituted by anionic r-donating groups have been studied regarding their electronic structure, ground state and stability. The results obtained have provided data indicative of the behaviour of such peculiar nitrenes, thus giving the opportunity to further understand, predict and tune the electronic properties of this class of reactive intermediates. 

These limitations, most urgently felt in solid state theory, have stimulated the search for alternative approaches to the many-body problem of an interacting electron system as found in solids, surfaces, interfaces, and molecular systems. Today, local density functional (LDF) theory (3-4) and its generalization to spin polarized systems (5-6) are known to provide accurate descriptions of the electronic and magnetic structures as well as other ground state properties such as bond distances and force constants in bulk solids and surfaces. 

Quantitative structure-physical property relationships (QSPR). There are two types of physical properties we must consider ground state properties and properties which depend on the difference in energy between the ground state and an excited state. Examples of the former are bond lengths, bond angles and dipole moments. The latter include infrared, ultraviolet, nuclear magnetic resonance and other types of spectra, ionization potentials and electron affinities. 

We shall now examine recent theoretical results concerning those ground state properties of metals and compounds which in the previous section, we have called bond and valence indicators. In this section, those properties are not taken as starting points for correlations aiming at the composition of the bond. They are instead the final point of electronic structure theories in which the different contributions to cohesion are analysed. 

Metallic bonding, due to the f-f overlap (as found, e.g. in NaCl-structure actinide compound), is consequently to be excluded in oxide systems, as already shown by Hill plots (see Chap. A). Brooks and Kelly have recently calculated ground state properties for UO2 in a LDA scheme by taking the hypothesis of an itinerant character of 5 f electrons (see Chap. C). This hypothesis leads to an excessive 5f attractive contribution to cohesion, leading to an equilibrium volume for UO2 35% lower than the experimental one. 

Systematic studies of well-defined materials in which specific structural variations have been made, provide the basis for structure/property relationships. These variations may include the effect of charge, hybridization, delocalization length, defect sites, quantum confinement and anharmonicity (symmetric and asymmetric). However, since NLO effects have their origins in small perturbations of ground-state electron density distributions, correlations of NLO properties with only the ground state properties leads to an incomplete understanding of the phenomena. One must also consider the various excited-state electron density distributions and transitions. 

More General Treatments of Electron Correlation in Polymers.—The introduction of excitonic states was just a simple example to show how one can go beyond the HF approximation to obtain correlated electron-hole pairs, whose energy level(s) may fall into the forbidden gaps in HF theory, and form the basis for interpretation of optical phenomena in semiconducting polymers. The schemes described until now for investigation of certain types of correlation effects (the DODS method for ground-state properties and the exciton-picture for excited states) are relatively simple from both the conceptual and computational points of view and they have been actually used at the ab initio level. It is evident, on the other hand, that further efforts are needed in polymer electronic structure calculations if we want to reach the level of sophistication in correlation studies on polymers which is quite general nowadays in molecular quantum mechanics. 

The present paper will first review shortly the way of performing Hartree-Fock (HF) calculations for ground state properties of polymers. By use of the Koopmans theorem, the corresponding HF density of states is of direct interest as an interpretative tool of XPS experiments. A practical way of correlating band structure calculations and XPS spectra is thus presented. In the last part, we illustrate the type of mutual enrichment which can be gained from the interplay between theory and experiment for the understanding of valence electronic properties. ... 

A brief review is given on the application of local density theory to the electronic structure of f-electron metals, including various ground state properties such as observed crystal structures, equilibrium lattice constants, Fermi surface topologies, and the electronic nature of known magnetic phases. A discussion is also given about the relation of calculated results to the unusual low energy excitations seen in many of these metals. 

Another model that describes the electronic structure of a system is provided by density functional theory (DFT). In DFT the electron density p of the system in the ground state plays the role of the many-electron wavefunction T in the wavefunction model because it uniquely defines all ground state properties of a system.An advantage of DFT is that T, which is a function of both spatial and spin coordinates of all electrons in the system, is replaced by a function that depends only on a position in Cartesian space p = p(r). The electron density can be obtained by using the variational principle... 

Electronic structure calculations have over the years almost solely been performed using state approaches. The basic approach has been to develop better and better methods to evaluate the wavefunction and/or the energy and other properties of the individual state. To a large extent the efforts have concentrated on ground-state properties. Clearly these methods have also dominated the literature and even very recent reviews of electronic structure calculations (Schaefer, 1984) maintain the view that only state function methods have yet demonstrated their usefulness in electronic structure calculations. It is the purpose of the present review to describe a different approach, the propagator methods. In these methods we compute state energy differences, transition probabilities and response properties directly without knowing the wavefunctions of the individual states. 

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