CORRELATED CAPPED SUBSYSTEM METHOD FOR THE CALCULATION OF SUBSTITUENTEFFECTS ON BOND-ENERGIES

The correlated capped small system strategy has been demonstrated to b e a valuable method for the calculation of bond energies and substitue nt effects on bond energies. By using the integrated molecular orbital -molecular orbital formulation, this strategy provides a means for int roducing electron correlation effects in cases where a correlated calc ulation on the entire system is not affordable, but both electron corr elation on a part of the system and substituent effects from another p art are required for obtaining accurate results. To apply this dual-le vel strategy to very large systems, one may consider various lower lev els for which the calculation on the whole system is affordable. In th e 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 CH(3)CH(2)X. T he 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 mor e accurate in the dual-level calculation than in either single-level c alculation (high level on capped small system or low level on entire s ystem); thus, integrating the levels is found to be a successful strat egy. Substituent effects are also more accurate with the dual-level st rategy.