Help on Secondary Orbital Interactions

 Dear CCLrs,
Does anyone know where I can find some information on Secondary Orbital Interactions (in Diels-Alder reactions and such)? What they are, how to compute them...? I have read a few papers on that (there is even discussion about whether they exist or not), in one of them they used something similar to the Mulliken charge analysis to account for this interactions, is there another (more accurate) way of computing them?
 I would greatly thank any piece of information on this subject.
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