A method of "natural population analysis" has been developed to calculate atomic charges and orbital populations of molecular wave functions in general atomic orbital basis sets. The natural analysis is an alternative to conventional Mulliken population analysis, and seems to exhibit improved numerical stability and to better describe the electron distribution in compounds of high ionic character, such as those containing metal atoms. We calculated ab initio SCF-MO wave functions for compounds of type CH 3 X and LiX (X = F, OH, NH 2 , CH 3 , BH 2 , BeH, Li, H) in a variety of basis sets to illustrate the generality of the method, and to compare the natural populations with results of Mulliken analysis, density integration, and empirical measures of ionic character. Natural populations are found to give a satisfactory description of these molecules, providing a unified treatment of covalent and extreme ionic limits at modest computational cost.

Ab initio and INDO LCBO-MO calculations are carried out on glyoxal, 1,3-butadiene, and acrolein in order to analyze the qualitative failure of INDO-like methods to describe conformation energies in these molecules. Following the method of Brunck and Weinhold for ethanelike systems we identify the principal bond-antibond interactions contributing to the glyoxal barrier, their dependence on dihedral angle and substituent, and their relative magnitudes as calculated by ab initio and INDO SCF-MO theory. We find a gross disparity in the INDO representation of n* interactions which leads to a grossly exaggerated estimate of the stability of gauche conformers in these molecules. These findings appear to have serious implications for the applicability of INDO-like theories to conformational problems in n-bonded molecules, including those of biological interest.

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