The hyperfine coupling constants of the X³∑^- states of NH

Abstract

The hyperfine coupling constants for the ³∑^- ground state of the NH molecule are determined by configuration interaction calculations whereby the influence of polarization functions as well as of the configuration space on the spin polarization mechanism is analysed. The dipolar part A_zz (N) and A_zz (H) can be obtained very reliably without much computational effort (A_zz (N) = -45·3 MHz and A_zz (H) = -62·3 MHz). The value for the isotropic contribution a _iso in the best AO basis and MRD-CI treatment is -64·5 MHz for H and 16·6 MHz for nitrogen compared to the corresponding experimental quantities of -66 MHz and 19 MHz respectively. Their determination depends on a subtle balance of the 1σ, 2σ and 3σ shell correlation description, whereby the cominant contribution to a _iso(H) results from the 2σ shell. It is shown that the often good agreement of a _iso values with experiment in a small basis single-double configuration interaction treatment results from a cancellation of two errors.