Comparison of the sum-over-states and finite perturbation theories of electrical polarizability and nuclear spin-spin coupling

The finite perturbation approach to electrical polarizability and nuclear spin-spin coupling is developed in a way which allows easy comparison with the conventional sum-over-states perturbation method. Both methods are used to calculate the electrical polarizability and nuclear spin-spin coupling of H2 and the different numerical results given by these two approaches are explained. These differences are essentially due to the fact that in FPT one cannot introduce singly-excited states into the perturbed wavefunction without at the same time introducing doubly-excited states in a restricted way.