Charge Distributions and Chemical Effects. XLI. Alkane Atomic Charges in Energy Calculations.

Abstract

Theoretical atomic charges of simple alkanes are examined for their validity in applications to physical problems, namely, in calculations of atomization energies. A generalization of Mulliken’s analysis, allowing for an uneven partitioning of CH overlap populations, achieves a basis set independent relative ordering of atomic charges. This ordering, reflecting the idea that any change in the electronic structure of carbon occurs most reluctantly, is precisely that required in energy calculations. The absolute magnitude of atomic charges, however, is basis set dependent. Extensive geometry and scale factor optimizations yield the following results (in 10^-3 e units) for the carbon net charge in ethane: 69.4 (ST0-3G), 55.1 (ST0-3G + CI), 42.8 (4-31G), and 37.8 (4-31G + CI). It appears that theoretical charge analyses converge toward the empirical result, 35.1 10^-3 e, provided they are carried out after configuration interaction involving reasonably large optimized basis sets