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 frameworks are only reliable if they do not depend either on a) the size of the perturbation, 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 studying the ¹J^FC(MN) and ²J^FC(MN) linear response behavior in a set of small molecules. It is shown how such a linearity depends on the size of the finite perturbation, the basis set employed, and the chosen nucleus site. The dependence of SCF convergence requirements on the size of the perturbation is also discussed. Calculations are carried out employing the B3LYP and B3PW91 hybrid functionals and results are compared with both state-of-the-art calculations and experimental values.