Correlated and Gauge Invariant Calculations of Nuclear Shielding Constants

Abstract

We discuss applications of a method proposed by Geertsen for gauge invariant calculations of NMR shielding constants. We apply it to small molecules where it is possible to use high quality basis sets. The method is derived using sum-over-states arguments without any reference to polarization propagator theory. We demonstrate the necessity of having basis set converged CHF/RPA results before firm conclusions about the importance of inclusion of higher order electron correlation in the shielding calculations can be made. It is shown how the shielding constants are modified when correlation is computed within the second order polarization propagator approach (SOPPA).