From configuration interaction to coupled cluster theory: The quadratic configuration interaction approach
Configuration interaction (CI) theory has dominated the first 50 years of quantum chemistry before it was replaced by many-body perturbation theory and coupled cluster theory. However, even today it plays an important role in the education of everybody who wants to enter the realm of quantum chemistry. Apart from this, full CI is the method of choice for getting exact energies for a given basis set. The development of CI theory from the early days of quantum chemistry up to our time is described with special emphasis on the size-extensivity problem, which after its discovery has reduced the use of CI methods considerably. It led to the development of the quadratic CI (QCI) approach as a special form of size-extensive CI. Intimately linked with QCI is the scientific dispute between QCI developers and their opponents, who argued that the QCI approach in its original form does not lead to a set of size-extensive CI methods. This dispute was settled when it was shown that QCI in its original form can be converted into a generally defined series of size-extensive methods, which however have to be viewed as a series of simplified coupled cluster methods rather than a series of size-extensive CI methods. © 2013 John Wiley & Sons, Ltd.