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| - A Journey from Generalized Valence Bond Theory to the Full CI Complete Basis Set Limit
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| - A qualitative examination of generalized valence bond pair correlation energies leads us to a quantitativerelationship (interference effect) between basis set truncation errors in MP2 energies and basis set truncationerrors in CCSD(T) energies. Thus, a knowledge of the MP2 complete basis set limit can be combined (forexample) with CCSD(T)/[5s4p3d2f/4s3p2d] calculations to estimate the CCSD(T) limit to within ±0.46 kcal/mol. Explicit MP2−R12 calculations are then compared to three extrapolation schemes employing cc-pVnZcorrelation consistent basis sets in an attempt to find an inexpensive route to the required MP2 limit. The firstemploys the N-1 asymptotic convergence of pair natural orbital (PNO) expansions to extrapolate to the completebasis set (CBS2) limit. The second employs (𝓁 + 1/2)-3 extrapolations of more than one MP2/cc-pVnZ calculationto estimate this MP2 limit. The third method combines the PNO extrapolations with a linear and thus size-consistent (𝓁 + 1/2)-3 extrapolation. This linear (𝓁 + 1/2)-3 extrapolation of first CBS2/cc-pVDZ and CBS2/cc-pVTZ then CBS2/cc-pVDZ and CBS2/cc-pVQZ energies gives the absolute MP2−R12 limit to within±0.86 and ±0.49 kcal/mol respectively for a test set of 12 small closed shell molecules, which represents anew level of accuracy for calculations fast enough to be routinely applied to molecules as large as naphthalene.Combining these MP2 limits with the interference corrected CCSD(T)/cc-pVDZ and CCSD(T)/cc-pVTZenergies respectively, gives the absolute CCSD(T) basis set limit to within ±1.74 and ±0.93 kcal/mol.
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