The history and evolution of configuration interaction

Abstract

The configuration interaction (CI) method dates back to the earliest days of quantum mechanics, and is the most straightforward and versatile approach for dealing with electron correlation. The earliest applications typically involved 2–10 terms, while modern molecular CI calculations often employ many millions of configuration state functions (CSFs). In addition to the enormous increase in computer power over the last fifty years, many theoretical developments have contributed to the evolution of the CI approach, including the development of efficient algorithmic tools for the various computational steps and the exploration and optimization of the choices of basis sets, orbitals, and the structure of the CI expansion. Among the milestones in these developments have been the introduction of efficient matrix eigenvalue methods, the introduction of multireference CI expansions, the formulation of various 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 and related methods have helped make direct CI calculations particularly efficient. Specialized computer programs for full CI calculations have become very efficient and are producing benchmark results which are extremely useful for evaluating other methodologies. Although it has lost ground to the very attractive coupled cluster methods, CI still has an important role to play in quantum chemistry.