Electron correlation described by extended geminal models: The EXGEM4 and EXGEM5 models

Within the framework of the general extended geminal model, two new approximate models EXGEM 4 and EXGEM 5 are introduced. The models are tested against full CI calculations on the water molecule for three different nuclear configurations and a full CI potential energy curve for the LiH molecule in the ground state. On the basis of these calculations, it is suggested that the models will yield electronic correlation energies with an accuracy of 1–2% of the corresponding full CI result.