A multireference CI determination of the isotropic hyperfine constants for first row atoms B–F

Abstract

The requirements for obtaining converged isotropic hyperfine constants, Aiso, in first row elements B–F are investigated with extended basis set CI wave functions composed of single and double excitations from a multiconfiguration reference space. Since the restricted Hartree–Fock unpaired spin density at the nucleus is zero for these elements, correlation effects account for the entire answer. Although the wave functions computed in the present study uniformly recovered 94%–98% of the empirical correlation energy, agreement with gas phase measurements of Aiso varied from a low of 55% on B to a high of 96% on N. Extension of these findings to the B2 (3Σg ) and H2 CO+ (2B2 ) molecules is also discussed. In order for small basis sets or very lightly correlated wave functions to produce good agreement with experiment, fortuitous cancellation of error between the negative core and positive valence contributions must occur. The present findings suggest relatively large basis sets and substantial correlation recovery are necessary if reliable core/valence balance is to be achieved.