Molecular orbital calculations, light method

We have shown in this paper that molecular orbital calculations at the ab initio level can be used to predict reliably the spectral transitions in silylenes, to evaluate the effects of substituents on the Si=Si multiple bond, to shed new light on existing experimental data and to direct future work towards the synthesis of novel isomers of disilenes. Although carbon and silicon are isoelectronic, multiple bonds to silicon differ dramatically from multiple bonds to carbon and analogies from carbon chemistry might therefore be entirely misleading when applied to silicon compounds. We believe that our studies have demonstrated the enormous power of modem computational methods and hope that this paper will prompt future theoretical studies and more importantly, theoretical-experimental collaborations in the field of organosilicon chemistry. 

Although this survey is primarily concerned with the interpretation of the results of molecular orbital calculations, it is apparent that a necessary prerequisite for such a scrutiny is the availability of a sufficiently extensive body of experimental data, with which comparisons may be made and the reliability of other theoretical predictions established. Consequently, it is pertinent here to examine the practical methods by which basic information about the bonding processes in sandwich species may be obtained, and how the ligand field parameters thus derived may shed light on the nature of the metal-ligand interactions. 

The potential value of the application of molecular orbital methods in colour chemistry is immense. In essence, the reason for this is that the methods enable, in principle, many of the light absorption properties of dyes, from a knowledge of their chemical structure, to be calculated with the aid of a computer. Thus, the colour properties of any dye whose structure may be drawn on paper may be predicted, with some expectation of accuracy, without the need to resort to devising a method for the... 

CCSDTQ (CC singles, doubles, triples, and quadruples) (75-75), CCSDTQP (CC singles, doubles, triples, quadruples, and pentuples) (7P), etc. approaches are far too expensive for routine applications. For example, the full CCSDTQ method requires iterative steps that scale as ( g(/i )is the number of occupied (unoccupied) orbitals in the molecular orbital basis). This scaling restricts the applicability of the CCSDTQ approach to very small systems, consisting of 2 - 3 light atoms described by small basis sets. For comparison, CCSD(T) is an nln procedure in the iterative CCSD steps and an nl procedure in the non-iterative part related to the calculation of the triples (T) correction. In consequence, it is nowadays possible to perform the CCSD(T) calculations for systems with 10-20 atoms. The application of the local correlation formalism (80-82) enabled SchOtz and Werner to extend the applicability of the CCSD(T) approach to systems with 100 atoms (53, 83, 84). 

Nowadays both models are still very much in use. Pauling s method is used to teach bonding and to visualize three-dimensional staictures. An extensive library of calculated molecular orbitals has accumulated and is helpful in explaining some aspects of another particular type of reactivity interactions of molecules with light, or spectroscopy. The most immediate triumph of quantum chemistry has been the explanation it offers for molecular spectroscopy. 

In this chapter, we therefore consider whether it is possible to eliminate spin-orbit coupling from four-component relativistic calculations. This is a situation quite different from that of more approximate relativistic methods where a considerable effort is required for the inclusion of spin-orbit coupling. We have previously shown that it is indeed possible to eliminate spin-orbit coupling from the calculation of spectroscopic constants [12,13]. In this chapter, we consider the extension of the previous result to the calculation of second-order electric and magnetic properties, i.e., linear response functions. Although the central question of this article may seem somewhat technical, it will be seen that its consideration throws considerable light on the fundamental interactions in molecular systems. We will even claim that four-component relativistic theory is the optimal framework for the understanding of such interactions since they are inherently relativistic. 

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