The limits of accuracy of the extended Koopmans’ theorem for electron affinities

Calculations using several basis sets of increasing sizes for electron attachment to LiH show that the error is ∼0.03 electron volts for the largest basis set when comparing the extended Koopmans' theorem (EKT) with differences in full configuration interaction calculations. The corresponding error in the ionization potential is ∼10-5 electron volts. The EKT electron affinities (EAs) of a set of small molecules were examined. The Dyson orbital pole strength, the Dyson orbital compactness, and the largest square of the Dyson orbital expansion coefficients were used to analyze the accuracy of the EKT for EAs.