SIZE-EXTENSIVITY CORRECTIONS IN CONFIGURATION-INTERACTION METHODS

Limited configuration interaction methods suffer from size-extensivity errors. The origin and behavior of these errors is discussed and new versions of single and multireference corrections are presented. Accuracy of the new and various other size-extensivity corrections used in the literature is discussed and compared in a series of model calculations and calculations on small molecules. None of the commonly used multireference corrections restores the size extensivity of multireference configuration interaction calculations. Our correction behaves correctly for the special case of a reference state composed from all singly and doubly excited configurations. Formulas for size extensivity corrections in the variational-perturbation method are given and discussed.