Phase-change rule

C, Anchors, Molecules and Independent Quantum Species II. The Phase-Change Rule and the Construction of Loops... 

We can now proceed to discuss the phase-change rule and its use to locate conical intersections. 

In this section, the systematic search for conical intersections based on the Longuet-Higgins phase-change rule is described. For conciseness sake, we limit the present discussion to Hiickel-type systems only, unless specifically noted otherwise. The first step in the antilysis is the determination of the LH loops containing a conical intersection for the reaction of interest. 

In this section, we apply the phase-change rule and the loop method to some representative photochemical systems. The discussion is illustiative, no comprehensive coverage is intended. It is hoped that the examples are sufficient to help others in applying the method to other systems. This section is divided into two parts in the first, loops are constructed and a qualitative discussion of the photochemical consequences is presented. In the second, the method is used for an in-depth, quantitative analysis of one system—photolysis of 1,4-cyclohexadiene. 

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