On the existence of free doubly negative molecular ions

We present a theoretical analysis on the existence of doubly negative molecular ions in the gas phase. A thorough investigation involves the discussion of two main problems: the stability of the system with respect to dissociation and the stability to the loss of an electron. An adequate treatment of both problems requires the choice of proper basis sets as well as the application of higher levels of theory. As an example the BeF ₄ ^2? dianion is studied in detail. Here we use configuration interaction and many-body Green's function approaches. Strong basis set dependencies in the calculations are observed which have no analogy in neutral systems.