The convergence of basis set contractions: A case study of the molecular hyperfine structure of NH214

Abstract

The convergence of basis set contractions is examined at the multireference configuration interaction level with respect to the 14N isotropic hyperfine coupling constant in NH2, a case that is very sensitive to the choice of the basis set and the degree of correlation recovery. The contraction schemes studied include the atomic natural orbital (ANO) approach and two Hartree–Fock-based contraction methods: Raffenetti’s general contraction method and the segmented method. Contractions of a (13s8p2d) primitive set ranging from [4s3p2d] to [8s3p2d] in the s space and from [7s3p2d] to [7s6p2d] in the p space are employed. It is found that the ANO contraction yields the smoothest and fastest convergence, although all three contractions converge to the uncontracted results. Thus, the ANO contraction starts to converge at [6s3p2d] while the other two converge at [7s4p2d]. The correlation recovery of the core electrons versus that of the valence electrons is also studied. In contrast to previous speculation, it is found that the correlation due to core electrons is recovered well with the simple choice of the most populated ANOs.