Bond properties from a localized MO-LCAO approach

Abstract

The problem of theoretically defining bond properties in a simple MO-LCAO method is tackled by partitioning and rearranging the energy expression for a single-determinant wave function built over bond orbitals. The latter are assumed to be formed each by two suitably hybridized MAOS. Orthogonality of the bond orbitals is ensured by a modification of Lowdin's orthogonalization procedure. Theoretical bond energies are defined, discussed, and estimated for a number of bonds. Bond Properties and Simple MO ModelsThe success of large-molecule computations both with ab initio and with allvalence electrons methods is still inducing many researchers to postpone work on the analysis of molecular computations. Among the few exceptions is the work of Gordon and England [l], based on a very deep and far-reaching study of Ruedenberg [2]. There the importance of defining and discussing theoretical bond properties to be compared with those of the classical theory of chemistry is stressed and justified. The importance of transferability, which is deeply connected with the bond concept [2,3], is also given due emphasis. However, some aspects of the general interpretational problem of molecular studies (and in particular the role of the minimal basis MO-LCAO scheme as a model for localization, 0--71 separation, etc.) deserve study along quite different although complementary lines. As an alternative to starting from an all-purpose semiempirical method based on as general a scheme as possible with simplifications essentially justified by numerical success, as is the case with extended Huckel and CNDO-like methods [4], one can imagine that the atomic orbital basis is already "prepared" so as to provide a description of a molecule in terms of bond orbitals, delocalized x systems, and so on. On that basis one may try to define bond quantities in much the same way as is done by a posteriori methods of localization [ 5 , 6 ] .Such a philosophy is interesting, in that it avoids the delicate transferability problem [3] by involving automatically defined bond properties, and that it has the possibility of suggesting when and why bond properties are not exactly trans-