Calculations of proton magnetic shielding constants in polyatomic molecules

Proton magnetic shielding constants in H2O, NH3, and CH4 molecules have been evaluated within the framework of coupled and uncoupled Hartree–Fock perturbation theories for the Fock–Dirac density matrix, employing several basis sets of gaugeless Gaussian functions. The results yielded by the largest basis sets are in good agreement with experiment, as well as other ab initio calculations. Some quantities, useful in testing the degree of gauge independence of computed screening constants and in evaluating this property for a gauge whatsoever, are also presented. Numerical tests demonstrate the basic soundness of the uncoupled approach to magnetic shielding proposed here, because of its higher accuracy with respect to other uncoupled schemes and its practicality for larger molecules.