Correlated and gauge origin independent calculations of magnetic properties

We have applied a gauge origin invariant method for the calculation of magnetizabilities and nuclear magnetic shielding constants to the triply bonded molecules CO, N2, HCN, CN- and HCCH. In particular, we have compared the basis set dependence of the sum-over-states expression for the diamagnetic term with the ground state average value expression and with the basis set dependence of the paramagnetic contribution. A systematic procedure for choosing an atomic basis set for the individual atoms was developed and applied to all molecules studied. Inclusion of p and d functions with large exponents was found important for the sum-over-states diamagnetic contribution to the nuclear magnetic shielding, whereas it was necessary to include diffuse d functions for the magnetizability. The effect of electron correlation on the diamagnetic and paramagnetic contributions to the magnetic properties was investigated within the second order polarization propagator approximation (SOPPA) and various coupled cluster polarization propagator approximations (CCDPPA/CCSDPPA). We find that the SOPPA gives a much smaller correlation contribution for the heavy atoms in these molecules than comparable MP2 and MC-RPA calculations. A large effect on the inclusion of coupled cluster single amplitudes suggests that the orbital relaxation terms might be quite important for magnetic properties of triply bonded molecules. Correlation effects are larger in small basis sets. This leads sometimes (N2 and CO) to much better agreement between theory and experiment using incomplete basis sets.