Ab initio charge distributions

Model Potentials for Organic Molecules Constructed from Ab Initio Charge Distributions and Empirical Fitting... 

The results of ab initio treatments of the (CH)j and (CH), isomers, and the cations and anions derived from them, have included bicyclo[2,l,0]pentane and norcaradiene, respectively, and SCF-MO calculations on (CH)g systems have appeared. The analysis of ab initio charge distributions in cyclohexane and methylcyclohexane has revealed a direct relationship between the carbon-13 chemical shift (5J and the relative net charge of the carbon atom qj ... 

Recent refinements on the atom-atom potential method include the development of accurate anisotropic model intermolecular potentials from ab initio electron distributions of the molecules. The non-spherical features in these charge distributions reflect features of real molecules such as lone pair and 7t-electron density, and therefore are much more effective at representing key interactions such as hydrogen bonding. 

Figure Al.3.22. Spatial distributions or charge densities for carbon and silicon crystals in the diamond structure. The density is only for the valence electrons the core electrons are omitted. This charge density is from an ab initio pseudopotential calculation [27].

Price S L, R J Harrison and M F Guest 1989. An Ab Initio Distributed Multipole Study of the Electrostatic Potential Around an Undecapeptide Cyclosporin Derivative and a Comparison with Point Charge Electrostatic Models. Journal of Computational Chemistry 10 552-567. 

Prior to 1965, all we had in our armoury were the a and it Hiickel theories, and a very small number of rigorous calculations designated ab initio (to be discussed later). The aims of quantum chemistry in those days were to give total energies and charge distributions for real molecules, and the seventh decimal place in the calculated properties of LiH. Practical chemists wanted things like reliable enthalpy changes for reactions, reaction paths, and so on. It should come as no surprise to learn that the practical chemists therefore treated theoreticians with scepticism. 

In the present work, we shall investigate the problem of the amount of correlation accounted for in the DF formalism by comparing the molecular electrostatic potentials (MEPs) and dipole moments of CO and N2O calculated by DF and ab initio methods. It is indeed well known that the calculated dipole moment rf these compounds is critically dependent on the level of theory implemented and, in particular, that introduction of correlation is essential for an accurate prediction [13,14]. As the MEP property reflects reliably the partial charges distribution on the atoms of the molecule, it is expected that the MEP will exhibit a similar dependence and that its gross features correlate with the changes in the value of dipole moment when switching from one level of theory to the other. Such a behavior has indeed been reported recently by Luque et al. [15], but their study is limited to the ab initio method and we found it worthwhile to extend it to the DF formalism. Finally, the proton affinity and the site of protonation of N2O, as calculated by both DF and ab initio methods, will be reported. 

In the 1920s it was found that electrons do not behave like macroscopic objects that are governed by Newton s laws of motion rather, they obey the laws of quantum mechanics. The application of these laws to atoms and molecules gave rise to orbital-based models of chemical bonding. In Chapter 3 we discuss some of the basic ideas of quantum mechanics, particularly the Pauli principle, the Heisenberg uncertainty principle, and the concept of electronic charge distribution, and we give a brief review of orbital-based models and modem ab initio calculations based on them. 

For the metal in the electrochemical interface, one requires a model for the interaction between metal and electrolyte species. Most important in such a model are the terms which are responsible for establishing the metal-electrolyte distance, so that this distance can be calculated as a function of surface charge density. The most important such term is the repulsive pseudopotential interaction of metal electrons with the cores of solvent species, which affects the distribution of these electrons and how this distribution reacts to charging, as well as the metal-electrolyte distance. Although most calculations have used parameterized simple functional forms for this term, it can now be calculated correctly ab initio. 

The charge densities and bond orders of the hypervalent bond of these telluranes 159, 160, and 161 were calculated by the ab initio RHF method on the basis of 3-21G. The results are shown in Figure 8. Interestingly, these ab initio calculations reveal that the positive charges at the hyperavalent bond 159 and 160 are distributed... 

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