ONIOM:  A Multilayered Integrated MO + MM Method for Geometry Optimizations and Single Point Energy Predictions. A Test for Diels−Alder Reactions and Pt(P(t-Bu)3)2 + H2 Oxidative Addition

The new ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) approach has been proposed and shown to be successful in reproducing benchmark calculations and experimental results. ONIOM3, a three-layered version, divides a system into an active part treated at a very high level of ab initio molecular orbital theory like CCSD(T), a semiactive part that includes important electronic contributions and is treated at the HF or MP2 level, and a nonactive part that is handled using force field approaches. The three-layered scheme allows us to study a larger system more accurately than the previously proposed two-layered schemes IMOMO, which can treat a medium size system very accurately, and IMOMM, which can handle a very large system with modest accuracy. This three-layered scheme has been applied to activation barriers for the Diels−Alder reaction of acrolein + isoprene, acrolein + 2-tert-butyl-1,3-butadiene, and ethylene + 1,4-di-tert-butyl-1,3-butadiene. In general, the results for b...