A COMPARISON OF UNRESTRICTED HARTREE-FOCK-BASED AND RESTRICTED OPEN-SHELL HARTREE-FOCK-BASED METHODS FOR DETERMINING THE MAGNETIC HYPERFINE PARAMETERS OF NO (X2-PI)

The magnetic hyperfine structure parameters of NO X 2PI have been determined through a variety of ab initio methods based on restricted and unrestricted Hartree-Fock zeroth order wave functions. Examples of the former include singles configuration interaction (CI), multireference CI, and averaged coupled pair functional theory. Examples of the latter include Moller-Plesset perturbation theory (through fifth order, with estimates to infinite order), coupled cluster methods, and quadratic CI (with approximate inclusion of triple and quadruple excitations). The performance of the various methods in reproducing the difficult-to-describe N-14 and O-17 isotropic hyperfine interactions is judged in light of both experimental data, where available, and estimated full CI values. The full CI limit was approached through a systematic sequence of ever-more-extensive, selected multireference CI wave functions that would, in principle, include the full CI as its final element. While the isotropic coupling constants were found to converge very slowly along this sequence, at least in comparison to other one-electron properties, the selected CI approach was efficient enough in its recovery of correlation effects to be used with large basis sets. The biggest calculation in the sequence of CI wave functions included over two million configurations. Energies and properties exhibited sufficient regularity to allow fitting with simple functional forms. The error arising from the lack of basis set completeness is estimated by comparison to fully numerical, partial-wave self-consistent field (SCF) and singles CI results. Effects due to vibrational motion are accounted for by numerical integration of the one-dimensional Schrodinger equation.