Localized atomic orbitals for atoms in molecules. I. Methodology

摘要

A method has been devised for the construction of localized atomic orbitals (LAOs) for atoms in molecules. The LAOs reflect the atomic hybridizations suggested by the localized molecular orbital (LMO) analyses of Edmiston and Ruedenberg. The procedure involves definition of a new function,L(AaVerbar;Aquest;), a measure of a particular type of exchange energy associated with the distribution between AOsaand quest; on atomA. The termsL(Aa)=L(AaVerbar;Aa) constitute an atomic localization sum,L(A), which is required to be a maximum in the LAO basis onA. The properties of the components ofL(A) are explored so as to achieve maximal computational efficiency. The maximization process is contingent upon an iterative sequence of 2times;2 rotations. In this paper it is shown that the method will yield satisfactory results for all closed shell free atoms plus for the F atom in the HF molecule. The minimal basis of LAOs on F in HF are partitioned as (1) a core orbital with a population of about two electrons, (2) three trigonally equivalent lone pair orbitals each with a population of about two electrons, and (3) a bonding orbital pointing toward H with a population of somewhat less than one electron. The results match both classical and LMO predictions. In general, the LAO and LMO methods, although independent, yield complementary pictures of localized valence and bonding, as is shown further in the second publication of this series. There are, however, several features of the LAO method for molecules which may make it an attractive alternative to the LMO process, including computational time requirements for larger systems and a much preferable description of the valence in C2.