A serious problem in the ab-initio Hartree-Fock calculation for extended systems is a singularity in the exchange contribution to the band energies (the "exchange pathology"). In the homogeneous electron gas this pathology is exactly cancelled by the correlation contribution arising from the sum of all timeordered ring diagrams ( RPA approximation). In this article we show that this cancellation is more general and does also occur in periodically extended systems. 0 1993 John Wiley & Sons, Inc.

We compute a number of properties for the 1 'S, 2 'S, and 2's states of helium as well as the ground states of H2 and H f using Variational Monte Carlo. These are in good agreement with previous calculations (where available), Electric-response constants for the ground states of helium, H2 and H are computed as derivatives of the total energy. The method used to calculate these quantities is discussed in detail.0 1992 John Wiley &Sons, inc.

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