Pericyclic processes show a wide diversity of factors which control their outcome. The symmetry characteristics of the wavefunctions, and the aromaticity that may or may not ensue, have been dealt with in the main section of this lecture course. But electronic symmetry does not always allow a single reaction path to be predicted; often two or more "allowed" pericyclic processes can be identified, and to further distinguish them, other factors such as steric effects, hydrogen bonding, etc must be taken into account. It is important to evaluate each effect quantitatively, since sometimes they might act in the same direction, sometimes in the opposite, and it is only by adding all the effects together that the final outcome can be predicted. The technique that allows this to be done is called "molecular modelling", and in this particular context, since electrons are involved, it is often referred to as quantum molecular modelling.
In the examples scattered throughout this lecture course, modelling was done at the DFT ωB97XD/6-311G(d,p) level, using the Gaussian 09 program. This technique is illustrated in the third year Computational modelling laboratory course.