Location of two seams in the proximity of the C2v ππ* minimum energy path of formaldehyde

Photochemical reactions rationalization is a key aspect for the understanding and setup of novel experiment and novel photoinitiated pathways. In this respect, the relationship between minimum energy paths over an excited-state and the intersection to lower potential energy surfaces is fundamental. In order to help the understanding of this relationship, in this study we present a novel kind of constraint for geometry optimizations, namely, an "orthogonality" constraint. Its possible applications are described. A complete example on how to retrieve the direct relationship between a minimum energy path over an excited-state potential energy surface and a conical intersection seam is given for C2v symmetry constrained formaldehyde. The advantages of using the novel constraint when rationalizing a (photo)chemical reaction are presented. © 2009 American Chemical Society.