6.18.1 Introduction

Selected CI methods seek to approximate full CI (FCI) energy, possibly in an active space, by including only the most important determinants for a particular problem. There are many flavors of selected CI going back to the 1970s. Q-Chem includes the adaptive sampling configuration interaction (ASCI) method. In the ASCI method, a trial CI wave function ${\psi }^k$ is iteratively improved by the inclusion of new determinants that are deemed important. The selection rule is derived from a consistency relationship among the coefficients in a CI expansion of the exact FCI wave function.