THE HISTORY AND EVOLUTION OF CONFIGURATION-INTERACTION

The configuration interaction (CI) method dates back to the earliest d ays of quantum mechanics, and is the most straightforward and versatil e approach for dealing with electron correlation. The earliest applica tions typically involved 2-10 terms, while modern molecular CI calcula tions often employ many millions of configuration state functions (CSF s). In addition to the enormous increase in computer power over the la st fifty years, many theoretical developments have contributed to the evolution of the CI approach, including the development of efficient a lgorithmic tools for the various computational steps and the explorati on and optimization of the choices of basis sets, orbitals, and the st ructure of the CI expansion. Among the milestones in these development s have been the introduction of efficient matrix eigenvalue methods, t he introduction of multireference CI expansions, the formulation of va rious corrections and modifications to overcome the major fault of CI, its lack of extensivity, and particularly the introduction of direct CI, which greatly increased the length of accessible CI expansions by eliminating the need to store the Hamiltonian matrix. Unitary group an d related methods have helped make direct CI calculations particularly efficient. Specialized computer programs for full CI calculations hav e become very efficient and are producing benchmark results which are extremely useful for evaluating other methodologies. Although it has l ost ground to the very attractive coupled cluster methods, CI still ha s an important role to play in quantum chemistry.