e sCI methods rely on the same principle as the usual con guration interaction (CI) approaches, except that determinants are not chosen a priori based on occupation or excitation criteria, but selected among the entire set of determinants based on their estimated contribution to the FCI wave function. Indeed, it has been noticed long ago that, even inside a prede ned subspace of determinants, only a small number of them signi cantly contributes. erefore, an on-the-y selection of determinants is a rather natural idea that has been proposed in the late 1960's by Bender and Davidson as well as Whi en and Hackmeyer. sCI methods are still very much under active development. e main advantage of sCI methods is that no a priori assumption is made on the type of electronic correlation. erefore, at the price of a brute force calculation, a sCI calculation is less biased by the user's appreciation of the problem's complexity.