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6.21 Adaptive Sampling Configuration Interaction Method

6.21.1 Introduction

(February 4, 2022)

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 ψk 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.