Basis set extrapolations are the foundation of many accurate thermochemical composite methods, allowing for much better accuracy at a much more favourable computational cost. Often, the extrapolation parameters are taken from the study by Neese and Valeev\(,^{1}\) and these parameters are often used with other methods, such as MP2 or double-hybrid DFT. Because I was irked by some related issues, I decided to look carefully at a few issues related to basis extrapolations: (i) Is Neese and Valeev’s stated method of simultaneously optimizing Hartree–Fock and CCSD(T) correlation energies justified; (ii) is it reasonable to optimize a single parameter for the CCSD(T) correlation energy; (iii) are the CCSD(T)-optimized parameters transferable to other methods, in particular double-hybrid DFT functionals; (iv) if the commonly used extrapolation methods (exponential–square root, exponential, power) for two basis sets can be reduced to a Schwenke-type form\(,^{2-3}\) is there any meaning to the different methods; (v) is there any benefit to expanding their fitting set to include second-row elements? In this talk, I will present my results that I are currently under consideration for publication.

1. Neese, F.; Valeev, E. F. Revisiting the Atomic Natural Orbital Approach for Basis Sets: Robust Systematic Basis Sets for Explicitly Correlated and Conventional Correlated ab initio Methods? \(\textbf{J. Chem. Theory Comput.}\) 7, 33–43 (2011).
2. Martin, J. M. L. A simple ‘range extender’ for basis set extrapolation methods for MP2 and coupled cluster correlation energies \(\textbf{AIP Conf. Proc.}\) 2040, 020008 (2018).
3. Schwenke, D. W. The extrapolation of one-electron basis sets in electronic structure calculations: How it should work and how it can be made to work. \(\textbf{J. Chem. Phys.}\) 122, 014107 (2005).