ELECTRON-CORRELATED CALCULATIONS OF THE NUCLEAR SHIELDING CONSTANTS AND SHIELDING POLARIZABILITIES FOR H2, N2, HF AND CO

The nuclear magnetic shielding sigma in a molecule is changed by the p resence of an external uniform electric field. This change may be expr essed in terms of the shielding polarizabilities sigma' and sigma''. V alues of sigma' and sigma'' have been evaluated for H-2, N2, HF and CO using a combination of the second order Moller-Plesset theory (MP2) a nd the finite-field perturbation theory. The effects of electron corre lation on the sigma' and sigma'' tensors for N2, HF and CO are seen to be sufficiently large that a higher-order theory than MP2 will be nec essary to account for them fully.