Correlated capped subsystem method for the calculation of substituent effects on bond energies

The correlated capped small system strategy has been demonstrated to be a valuable method for the calculation of bond energies and substituent effects on bond energies. By using the integrated molecular orbital-molecular orbital formulation, this strategy provides a means for introducing electron correlation effects in cases where a correlated calculation on the entire system is not affordable, but both electron correlation on a part of the system and substituent effects from another part are required for obtaining accurate results. To apply this dual-level strategy to very large systems, one may consider various lower levels for which the calculation on the whole system is affordable. In the present work, we examine the behavior of several such lower levels, in particular semiempirical molecular orbital methods based on neglect of diatomic differential overlap, ab initio Hartree-Fock calculations with small basis sets, and density functional theory. The methods are tested for calculating C-H bond energies and substituent effects in a series of substituted ethanes with the general formula CH₃CH₂X. The entire systems considered here are ethane (X = H), propane (X = CH₃), ethanol (X = OH), ethylamine (X = NH₂), and fluoroethane (X = F). For 11 of the 13 dual-level methods that we tested, bond energies are more accurate in the dual-level calculation than in either single-level calculation (high level on capped small system or low level on entire system); thus, integrating the levels is found to be a successful strategy. Substituent effects are also more accurate with the dual-level strategy.