Charge distributions and chemical effects. XLVII. Density matrix contribution to the charge distribution in hydrocarbons, arising from singly excited configurations in CI calculations

The involvement of excited configurations in Mulliken charge analyses is examined for ethylene and acetylene, using an optimized 4-31G basis. The net charges of carbon, −346.4 × 10−3 (C2H4) and −335.3 × 10−3 e (C2H2) at the SCF level, are reduced to −269.9 × 10−3 and −271.2 × 10−3 e, respectively. Double excitations appear to contribute little to these corrections. In acetylene, three single σ → σ* type excitations are responsible for ~83% of the charge correction whereas, as expected, the role of π → π* type excitations is small. Similarly, four σ → σ* configurations account for ~76% of the correction in ethylene. These effects are particularly important in comparisons with alkanes, whose charges are relatively little affected by CI corrections. Theoretical charges obtained from CI calculations appear to converge toward their empirical counterparts in a generalization of Mulliken's scheme, which allows for an uneven partitioning of CH overlap populations. Keywords: charge density, configuration interaction.