Localized atomic orbitals for atoms in molecules

A method is devised for the construction of localized atomic orbitals (LAOs) for atoms in molecules. The procedure involves definition of a function, L(Aa,Aa bar), a measure of a particular variety of exchange energy associated with the distribution between AOs a and a bar on atom A. The terms L(Aa) = L(Aa,Aa) constitute an atomic localization sum., L(A), which is required to be a maximum in the LAO basis on A. The properties of the components of L(A) are explored so as to achieve greatest computational efficiency. The maximization process is contingent upon an iterative sequence of 2 x 2 rotations. For the molecules tested, LAOs partition into sets of core, bonding, and lone pair orbitals, thereby lending a rigorous quantum mechanical interpretation to classical and pseudo-classical notions of atomic hybridization. LAOs are found to be extremely transferable among similar systems which allows for the development of generalized LAOs to account for average hybridization of many chemically important systems. The Mulliken populations of LAOs correspond quite closely to classical concepts. Finally, LAO studies of the barrier to internal rotation in ethane are discussed.