Abstract
We consider various possibilities for the uncorrelated reference for the calculation of properties. According to the philosophy of Lowdin, to 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, however, 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 for the uncorrelated reference state. In this method the reference state 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.
Published Version
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