Calculation of hyperfine coupling constants of the ground state X 3Σ− of NH and B2

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Abstract
Following a systematic examination of basis set and electron correlation effects, accurate hyperfine coupling constants have been determined for the X 3Σ− states of NH and B2 using the multiconfiguration self-consistent-field (MCSCF) restricted–unrestricted (RU) response function approach. These species were chosen for study because their unpaired electrons reside in π orbitals; so at the single configuration self-consistent-field (SCF) approach, they display zero hyperfine coupling constants. The approach advocated here has been tested successfully on σ-radical species with unpaired electrons occupying σ orbitals; this work represents the extension to π-radical species which are expected to be more difficult cases. In designing the atomic orbital basis sets, effects of uncontraction of the orbitals (to permit maximal flexibility especially in describing electron density near nuclei) and of addition of diffuse and tight functions were taken into account. Our final bases give hyperfine coupling constants that agree with numerical Hartree–Fock (HF) and with numerical complete active space valence (CASV) MCSCF results, which indicates that our basis sets are accurate enough to be used in further studies that treat electron correlation more accurately. For dealing with electron correlation in a manner that, based on our past experience, could provide the requisite over all accuracy in the final coupling constants, the CASV configuration spaces were systematically extended to larger CAS (complete active space) spaces using natural orbital occupation numbers to determine which orbitals to include in active spaces for each symmetry. Our final results compare favorably with the available experimental data. The results show that the hyperfine coupling constant for B in B2 and N in NH results from a near cancellation of large and opposite signed core and valence contributions.