Electronic structure of the [MgO 3 ] + cation

Accurate ab initio calculations are performed to investigate the stable isomers of [MgO 3 ] + and its lowest electronic states at both molecular and asymptotic regions. The calculations are done using large basis sets and configuration interaction methods including the complete active space self-consistent field, the internally contracted multi-reference configuration interaction, the standard coupled cluster (RCCSD(T)) approaches and the newly implemented explicitly correlated coupled cluster method (RCCSD(T)-F12). The presence of three stable forms is predicted: a cyclic global minimum c-MgO 3 + , which is followed by a quasi-linear isomer, l2-MgO 3 + . A third isomer of C s symmetry (l1-MgO 3 + ) is also found. Moreover, we computed the one-dimensional cuts of the six-dimensional potential energy surfaces of the lowest doublet and quartet electronic states of [MgO 3 ] + along the R MgO and R OO stretching coordinates covering both the molecular and the asymptotic regions. These curves are used later for discussing the metastability of this cation and to propose plausible mechanisms for the Mg + + O 3 atmospherically important ion-molecule reaction and related reactive channels.