Complete to second-order ab initio level calculations of electronic g-tensors

The electronic g-tensors for NO2, CO+ and H2O+ are calculated at the restricted open-shell Hartree-Fock (ROHF) level using the Rayleigh-Schrodinger perturbation approach. All known first- and second-order contributions have been evaluated, including the relativistic mass correction, one- and two-electron spin Zeeman gauge correction terms, and one- and two-electron second-order terms. Substantial code development has been necessary, including an integral routine for computing the two-electron spin-Zeeman gauge correction term. Calculations have been done using triple zeta and quadruple zeta basis sets with additional polarization and semi-diffuse functions. Effective gauge invariance is obtained by placing the gauge origin at the molecule's electronic charge centroid. Excited state energies in the sum-over-states expansion are expressed using determinantal energies, thus avoiding the non-uniqueness of ROHF eigenvalues. Our results successfully reproduce trends in gas phase g-shifts (Delta g = g - g(e)). However, discrepancies between our calculated g-shifts and experimental ones, sometimes on the order of 50%, point to the need for a correlated treatment.