On the convergence of FPT-DFT calculations of the Fermi contact contribution to NMR coupling constants

Calculations of the Fermi contact term of NMR couplings, J(FC)(MN), using a single FC perturbation within the Finite Perturbation Theory-DFT framework s are only reliable if they do not depend either on a) the size of the pert urbation, and b) whether the perturbation is placed at the site of the M or N nucleus. In this short communication these two points are addressed by s tudying the (1)J(FC)(MN) and (2)J(FC)(MN) linear response behavior in a set of small molecules. It is shown how such a linearity depends on the size o f the finite perturbation, the basis set employed, and the chosen nucleus s ite. The dependence of SCF convergence requirements on the size of the pert urbation is also discussed. Calculations are carried out employing the B3LY P and B3PW91 hybrid functionals and results are compared with both state-of -the-art calculations and experimental values.