Efficient calculation of electron paramagnetic resonance $g$-tensors by multireference configuration interaction sum-over-state expansions, using the atomic mean-field spin–orbit method

Using the multireference configuration interaction method due to Grimme and Waletzke, combined with the atomic mean-field approximations for the efficient calculation of spin–orbit matrix elements, the $g$-tensors in second-order perturbation theory have been calculated for the main group radicals $CO+,$ CN, BO, BS, MgF, AlO, $O2,$ HCO, $H2O+,$ $NO2,$ $CO2−,$ $NF2,$ $NO22−,$ $O3−,$ $ClO2,$ and $H2CO+,$ and for the transition metal compounds ZnH, ZnF, and $TiF3,$ using explicit sum-over-state expansions for up to 20 excited states. In most cases, a valence triple-zeta basis set with polarization functions has been employed. It is shown that the addition of diffuse functions to this basis set does not improve the $g$-tensor results, and in several instances leads to slower convergence of the sum-over-state expansion. The calculated $g$-tensors are in good agreement with experimental values, and with our previous multireference configuration interaction results available for 9 of the 19 radicals. Our results are shown to be equivalent to, or better than, values obtained by other theoretical methods. Examples of radicals for which $g$-tensor calculations presented problems in the past are AlO and $TiF3.$ For AlO, we obtain $Δg⊥=−1530 ppm$ (parts per million), compared with an experimental value of −1900 ppm in Ne matrix. Using the SVP (valence double-zeta plus polarization) basis set, $Δg⊥$ of $TiF3$ is calculated to be −115.3 ppt (parts per thousand), compared with experimental values of −111.9 and −123.7 ppt.