Knowledge Bank

Once the necessary integrals are available. Roothaan’s SCF (self-consistent-field) method for approximating the electronic structures of molecules in terms of LCAO MO’s (molecular orbitals approximated by a linear combination of molecular orbitals) becomes an extremely promising procedure for obtaining detailed insight into the electronic structures of molecules and radicals, and of computing degrees of hybridization, ionization potentials, dipole moments etc., especially when generalized to include configuration interaction. The first successful computations by the LCAO-SCF method considered $\pi$ electrons only, in an assumed Goeppert-Meyer-Sklar field due to the remaining electrons. Next Mulligan ($CO2$) and very recently Sahni (CO) have published results of LCAO-SCF computations including all but the ls electrons explicitly in the calculation, with very interesting results, although the values of many of the integrals were approximated instead of computed. Also, Ellison and Shull have very recently published similar results on $H2O$ with many of the integrals approximated, but with all electrons included. In a paper following this one (paper A3), Scherr will report the results of calculations on $N2$ including all electrons, and with no approximations in the integrals. Sahni (work in course of publication) has recently already made similar calculations on BH. All calculations so far made have been only for the equilibrium internuclear distance, and without configuration interaction except for 2s, 2p hybridization. The present report will illustrate, in terms of examples, how the results of LCAO-SCF calculations can be used to obtain various kinds of breakdowns of the electronic population of a molecule. A breakdown in terms of populations in the various AO’s (1s, 2s, 2px, 2py, 2pz) of each atom in a molecule seems to display the extent of 2s, 2p hybridization and (in polar molecules) of transfer of charge between atoms. Another breakdown into atomic and resonance overlap populations provides an index of the extent and distribution of covalent bonding (or antibonding) within the molecule. The overlap indices plus Coulomb and possibly other smaller terms may form a basis for a reliable, largely theoretical, but partly empirical systematics of bond or dissociation energies, which are particularly difficult to compute reliably by purely theoretical methods.