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

The magnetic hyperfine structure parameters of NO X 2PI have been dete rmined 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 latt er include Moller-Plesset perturbation theory (through fifth order, wi th estimates to infinite order), coupled cluster methods, and quadrati c CI (with approximate inclusion of triple and quadruple excitations). The performance of the various methods in reproducing the difficult-t o-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 seque nce of ever-more-extensive, selected multireference CI wave functions that would, in principle, include the full CI as its final element. Wh ile the isotropic coupling constants were found to converge very slowl y along this sequence, at least in comparison to other one-electron pr operties, the selected CI approach was efficient enough in its recover y of correlation effects to be used with large basis sets. The biggest calculation in the sequence of CI wave functions included over two mi llion configurations. Energies and properties exhibited sufficient reg ularity to allow fitting with simple functional forms. The error arisi ng from the lack of basis set completeness is estimated by comparison to fully numerical, partial-wave self-consistent field (SCF) and singl es CI results. Effects due to vibrational motion are accounted for by numerical integration of the one-dimensional Schrodinger equation.