A b i n i t i o calculation of hyperfine splitting constants of molecules

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Abstract
Hyperfine splitting (hfs) constants of molecules, methyl, ethyl, vinyl, allyl, cyclopropyl, formyl, O3−, NH2, NO2, and NF2 radicals have been calculated by the pseudo‐orbital (PO) theory, the unrestricted HF (UHF), projected UHF (PUHF) and single excitation (SE) CI theories. The pseudo‐orbital (PO) theory is based on the symmetry‐adapted‐cluster (SAC) expansion proposed previously. Several contractions of the Gaussian basis sets of double‐zeta accuracy have been examined. The UHF results were consistently too large to compare with experiments and the PUHF results were too small. For molecules studied here, the PO theory and SECI theory gave relatively close results. They were in fair agreement with experiments. The first‐order spin‐polarization self‐consistency effect, which was shown to be important for atoms, is relatively small for the molecules. The present result also shows an importance of eliminating orbital‐transformation dependence from conventional first‐order perturbation calculations. The present calculations have explained well several important variations in the experimental hfs constants.