FINITE PERTURBATION-CONFIGURATION INTERACTION CALCULATIONS OF NUCLEAR SPIN-SPIN COUPLING-CONSTANTS .1. 1ST ROW HYDRIDES AND THE HYDROGEN MOLECULE

The finite perturbation scheme for calculations of nuclear spin-spin couplings based on the numerical computation of the second derivative of the total energy is described and compared to the earlier approach which uses the first derivative of the spin density matrix. The method is applied, at the Hartree-Fock and configuration interaction level of approximation, to calculations of the Fermi contact contribution to proton-proton and proton-first-row-atom coupling constants in H2, HF, H2O, NH3, and CH4. The analysis and the numerical calculations prove that for the many-electron systems the finite perturbation method is superior to the conventional second order perturbation theory approach if the same type of CI expansion is used. For the hydrogen molecule the two methods have to give identical results. The finite perturbation calculations for the first row hydrides show the following: (i) Upon going from the Hartree-Fock to the CI level the contact contributions are reduced by about 50% for the proton-proton couplings and by about 25% for the proton-first-row-atom couplings. (ii) The differences in the proton-proton couplings within the series of the first row hydrides cannot be explained without including the noncontact mechanisms. (iii) For the hydrogen fluoride, the noncontact mechanisms may contribute as much as 40% of the observed coupling. (iv) When appropriate corrections for the noncontact mechanisms are included the agreement between the calculations and experiments becomes reasonably good. © 1979 American Institute of Physics.