Paramagnetic shielding term approximate calculation

The complexity of the paramagnetic term is more apparent now. Not only do we require knowledge of the ground state wavefunction but we also need to know all the excited states and their associated energies, if we are to do the calculation exactly, even in the perturbation theory approximation. Clearly, these requirements cannot be met, and other approximations must be employed. Typically one starts at the coupled perturbed Hartree-Fock level, and for many years this was the sole approach. The inclusion of correlation from the Hartree-Fock base is complex but possible, and some important results have been obtained. The other approach, presently gaining favor, is to use density functional theory, i which includes correlation from the beginning, and then employ perturbation theory to treat the second-order shielding effect. 

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