Transition Structures on a Potential Energy Surface

The success of simple theoretical models in determining the properties of stable molecules may not carry over into reaction pathways. Therefore, ab initio calculations with larger basis sets may be more successful in locating transition structures than semi-empir-ical methods, or even methods using minimal or small basis sets. 

Nevertheless, this area is one of the richest fields for chemical theory, with great potential to deepen our understanding of the way chemical reactions occur. 

---

An IRC calculation examines the reaction path leading down from a transition structure on a potential energy surface. Such a calculation starts at the saddle point and follows the path in both directions from the transition state, optimizing the geometry of the molecular system at each point along the path. In this way, an IRC calculation definitively connects two minima on the potential energy surface by a path which passes through the transition state between them. 

---