Calculations of magnetic susceptibility of polyatomic molecules

The magnetic susceptibility of H2O, NH3, and CH4 molecules has 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 provided by the largest basis sets are in good agreement with experiment as well as with other ab initio calculations. Quantities necessary to assess the degree of gauge independence of the susceptibility and to evaluate this property for an arbitrary gauge origin are also reported. The reliability of the uncoupled approach suggested here is systematically analyzed by investigating the upper bounds to the error affecting the geometric approximation to the paramagnetic susceptibility. The results demonstrate the practicality of the simple and inexpensive uncoupled scheme with respect to the cumbersome coupled method.