Systematic Analysis of Bond Energies Calculated by the Integrated Molecular Orbital−Molecular Orbital Method

This paper tests the ability of the integrated molecular orbital−molecular orbital (IMOMO) method to include the effects of extended basis sets and higher-order electron correlation on bond energies by treating only a capped subsystem of a large molecule at a high level and integrating this with a lower-level calculation on the entire system. First, our results show that improving the basis on the capped subsystem is an excellent way to improve the bond energy without the expense of using a large basis set for the whole system. In a second study, we show that correlated calculations on a capped subsystem with the Hartree−Fock (HF) approximation for the entire system yield results that are within 1 kcal/mol of CCSD(T)/6-31G(d,p) accuracy for a set of molecules involving first-row atoms. For chloroethane, though, the HF level is inadequate for such a treatment, and the calculations have an error of about 3 kcal/mol with respect to a full CCSD(T)/6-31G(d,p) calculation. However, for this case, using even a low level of electron correlation, such as Møller−Plesset second-order perturbation theory (MP2), for the entire system makes it possible to achieve high accuracy. A final set of results show how well we can get an accuracy comparable to MP2/6-31G(d,p) by using only Hartree−Fock calculations for the entire system. Such an integrated strategy, using a double-zeta basis set, is good enough for obtaining MP2/6-31G(d,p) accuracy within 1 kcal/mol for all the molecules investigated except difluoroethane and chloroethane.