BENCHMARK CALCULATIONS WITH CORRELATED MOLECULAR WAVE-FUNCTIONS .10. COMPARISON WITH EXACT MP2 CALCULATIONS ON NE, HF, H2O, AND N-2

The convergence of the MP2 valence correlation energy and pair energie s for the correlation consistent basis sets has been investigated. Ne, HF, H2O, and N-2 were studied. For all of these molecules, accurate M P2 correlation and pair energies are available from the recent MP2-R12 . calculations of W. Klopper [J. Chem. Phys. 102, 6168 (1995)]. The ma gnitudes of the calculated MP2 valence correlation and pair energies a re found to increase systematically with increasing basis set size, wi th the cc-pV6Z basis set yielding 97.4%-98.3% of the MP2 valence corre lation energy. A detailed analysis of the results for Ne reveals that the error due to truncation of the radial functions in the cc-pV6Z set is comparable to that due to neglect of higher angular momentum funct ions. Procedures for extrapolating the results to the complete basis s et limit have also been investigated. (C) 1997 American Institute of P hysics.