An approximate infinite order perturbation theory prescription for isotropic NMR chemical shieldings

Data in the literature from NMR chemical shielding calculations at Hartree-Fock, MP2, MP3, and MP4 levels using large basis sets suggest that the ratio of the various perturbation order contributions may be approximately constant with a value of -0.5. Under this assumption one may sum the infinite perturbation series so that, with a knowledge of Hartree-Fock and MP2 results, the infinite order shielding may be estimated. Applying this method to cited literature calculations provides NMR shieldings that are as accurate as available MP4 and CCSD calculations and in good agreement with experiment.