Finite-field many-body perturbation theory of the Fermi contact nuclear-spin coupling

The Fermi contact term of the nuclear-spin---spin coupling in the HD molecule is calculated using finite-field many-body perturbation theory (FF MBPT). The one-electron states are generated by the Hartree-Fock potential in the presence of the Fermi contact interaction. The procedure through third order in electron correlation gives a value for the Fermi contact term of 37.2 Hz in fair agreement with the best estimated value of 40.0 Hz at the equilibrium internuclear distance. The result suggests that the FF MBPT approach provides a highly efficient and systematic method for the calculation of the Fermi contact term. A possible origin of the general overestimate by the coupled Hartree-Fock method is also discussed.