The Origin of the Halogen Effect on Reactivity and Reversibility of Diels–Alder Cycloadditions Involving FuranWe are grateful to the National Science Foundation and the Partnerships for Advanced Computational Infrastructure (PACI) for financial support of this research. The computations were performed on the National Science Foundation Terascale Computing System at the Pittsburgh Supercomputing Center (PSC) and on the UCLA Academic Technology Services (ATS) Hoffman Beowulf cluster. We are grateful to Albert Padwa and Michael E. Jung for discussions.