Benchmark calculations with correlated molecular wave functions. X. Comparison with “exact” MP2 calculations on Ne, HF, H2O, and N2

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Abstract
The convergence of the MP2 valence correlation energy and pair energies for the correlation consistent basis sets has been investigated. Ne, HF, H 2 O , and N 2 were studied. For all of these molecules, accurate MP2 correlation and pair energies are available from the recent MP2-R12 calculations of W. Klopper [J. Chem. Phys. 102, 6168 (1995)]. The magnitudes of the calculated MP2 valence correlation and pair energies are found to increase systematically with increasing basis set size, with the cc-pV6Z basis set yielding 97.4%–98.3% of the MP2 valence correlation 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 functions. Procedures for extrapolating the results to the complete basis set limit have also been investigated.