A Journey from Generalized Valence Bond Theory to the Full CI Complete Basis Set Limit

Abstract

A qualitative examination of generalized valence bond pair correlation energies leads us to a quantitative relationship (interference effect) between basis set truncation errors in MP2 energies and basis set truncation errors in CCSD(T) energies. Thus, a knowledge of the MP2 complete basis set limit can be combined (for example) 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-pVnZ correlation consistent basis sets in an attempt to find an inexpensive route to the required MP2 limit. The first employs the N-1 asymptotic convergence of pair natural orbital (PNO) expansions to extrapolate to the complete basis set (CBS2) limit. The second employs (𝓁 + 1/2)-3 extrapolations of more than one MP2/cc-pVnZ calculation to 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 (...