ON THE UNCORRELATED REFERENCE FOR CALCULATION OF PROPERTIES

We consider various possibilities for the uncorrelated reference for t he calculation of properties. According to the philosophy of Lowdin, t o whom this volume is dedicated, the uncorrelated reference state for response properties ought to be taken as the unperturbed Hartree-Fock state for properties of all orders in the applied field. Frequently, h owever, it is operationally convenient to use the coupled Hartree-Fock like results as a standard of comparison for second- and higher-order properties, even though this is not consistent with Lowdin's choice f or the uncorrelated reference state. In this method the reference stat e is the perturbed Hartree-Fock state. Numerical examples demonstrate a rather large difference between the two uncorrelated references. We consider the pros and cons of each choice.