AN APPROXIMATE INFINITE-ORDER PERTURBATION-THEORY PRESCRIPTION FOR ISOTROPIC NMR CHEMICAL SHIELDINGS

Data in the literature from NMR chemical shielding calculations at Har tree-Fock, MP2, MP3, and MP4 levels using large basis sets suggest tha t the ratio of the various perturbation order contributions may be app roximately constant with a value of -0.5. Under this assumption one ma y sum the infinite perturbation series so that, with a knowledge of Ha rtree-Fock and MP2 results, the infinite order shielding may be estima ted. Applying this method to cited literature calculations provides NM R shieldings that are as accurate as available MP4 and CCSD calculatio ns and in good agreement with experiment.