... of Lewis acid catalyst BF3 on the mechanism of hetero Diels- Alder reactions Calculations show that effects of catalyst are complicated effects are different for different pathways Chapter Theoretical ... zero, it is called anti bond orbital (σAB*) The normalized and orthogonalized hA is called natural hybrid orbital (NHO, hA’) σAB can always be written as caha’ + cbhb’ and σAB* as caha’ - cbhb’ ... Petersson, G A. ; Montgomery, J A. ; Raghavachari, K.; Al-Laham, M A. ; Zakrzewski, V G.; Ortiz, J V.; Foresman, J B.; Cioslowski, J.; Stefanov, B B.; Nanayakkara, A. ; Challacombe, M.; Peng, C Y.; Ayala,...