Electromagnetic nuclear shielding tensors and their relation to other second-order properties. A study of the ammonia molecule

The results of an accurate calculation of second-order properties of the ammonia molecule are reported, with particular attention to nuclear electromagnetic shielding tensors. These quantities enable one to evaluate the actual fields at the nuclei of a molecule perturbed by external radiation. The present theoretical study provides an extended numerical test on the mutual relationships among different properties. A theoretical scheme, based on force and torque gauges for the transition moments, has been applied to partition the molecular polarizabilities, the paramagnetic susceptibilities, and other properties into atomic terms transferable from molecule to molecule.