Atomic orbitals, chemical conventions

In writing the conventional Lewis structures for molecules, we assume that a covalent chemical bond between two atoms involves sharing of a pair of electrons, one from each atom. Figure 6-5 shows how atomic orbitals can be considered to be used in bond formation. Here, we postulate that a single bond is formed by the pulling together of two atomic nuclei by attractive forces exerted by the nuclei for the two paired electrons in overlapping atomic orbitals. 

Although Conrad et al. have frequently justified their results in terms of conventional lattice-defect theory (including the size-misfit and modulus-defect formalisms), they have gone on to consider the effects of chemical interaction between the solute and solvent atoms. In doing so, the interaction mechanism was deduced, with the aid of atomic-orbital theory, to take the form of covalent bonding between the interstitial atom and the... 

The dimension of the secular determinant for a given molecule depends on the choice of basis set. EHT adopts two critical conventions. First, all core electrons are ignored. It is assumed that core electrons are sufficiently invariant to differing chemical environments that changes in their orbitals as a function of environment are of no chemical consequence, energetic or otherwise. All modern semiempirical methodologies make this approximation. In EHT calculations, if an atom has occupied d orbitals, typically the highest occupied level of d orbitals is considered to contribute to the set of valence orbitals. 

Fig. 32 shows on the left a conventional, localized domain model of the electronic environment of an atom that satisfies the Octet Rule. Each domain is occupied by two electrons. It is well known, however, that the assumption of two electrons per orbital is unnecessarily restrictive 27>122>. Better energies are obtained in quantum mechanical calculations if different orbitals are used for electrons of different spins, a fact first demonstrated in quantitative calculations on helium by Hylleraas 123> and Eckart 124>. Later, this "split-orbital method was applied to 71-electron systems 27,125) Its general application to chemical systems has been developed by Linnett 126>. 

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