Pitfalls for the Frozen-Core Approximation: Gaussian-2 Calculations on the Sodium Cation Affinities of Diatomic Fluorides

Abstract

The Gaussian-2 (G2) total energies for species having the formula Na(FX)+ [X = H, Li → F, or Na → Cl], calculated using conventional and widely used ab initio computational program suites, show serious deficiencies which are attributable to two different effects. Firstly, for the sodium-ion adducts of almost all of the covalent fluoridesnamely HF, BF, CF, NF, OF, F2, SF, and ClFthe orbital corresponding most closely to the fluorine 2s orbital possesses a more negative eigenvalue than the set of three orbitals corresponding most closely to the sodium 2px, 2py, and 2pz orbitals, so that routine selection of the “frozen-core” option (in the single-point calculations involved in determining the G2 total energy) leads to an inappropriate correlation space. Secondly, for the sodium-ion adducts of several fluoridesmost notably, but not solely, the ionic fluorides LiF, NaF, MgF, and AlFthere is very significant mixing of the fluorine 2s and sodium 2pz orbitals, with the result that the G2 frozen-core calculations...