Combined AM1/MM3 computations on organic systems: the Diels–Alder reaction as a test case

A combined AM1/MM3 method for computations in organic chemistry has been implemented and applied to the study of Diels–Alder reactions using the localised self-consistent field approach. In the case of acrylic esters, we show that the O–R bond is adequate for separating the quantum and classical moieties. Results for the reaction-energy profile, Lewis-acid catalytic effect and diastereofacial selectivity are presented. The agreement with full quantum computations is in general good. In addition, we show that AM1/MM3 computations overcome the well-known deficiencies of semi-empirical core–core repulsion functions.