Local correlation models

Local correlation models that meet the requirements of a theoretical model chemistry are discussed. Two types of models are considered. The first class uses a valence active space that associates one correlating orbital for each occupied valence orbital. In these models the fundamental quantity is the electron pair; even the simplest local approximation (perfect pairing) exactly treats an isolated electron pair in this active space. The second class of models uses no active space. In this case we argue that the most appropriate fundamental quantity is the atom; even the simplest local approximation (atoms-in-molecules) exactly treats an isolated atom. © Springer-Verlag Berlin Heidelberg 2003.