On the metastability of the1Sigma+gground state of He2+2and Ne2+2: a case study of binding metamorphosis

The transition from stable to metastable and finally to unstable situations for increasing nuclear charge Z is studied in the adiabatic approximation for the 1 Sigma + g ground state of homonuclear diatomic molecular systems with two or eighteen electrons. Allowing non-integer values for the parameter Z and varying it in small steps, critical values for stability Z c and Z m are determined by means of extensive and accurate ab initio calculations. In addition, the behaviour of various other quantities (equilibrium distances, lowest bound or resonance levels of vibrational motion) is monitored during the transition. The authors' results confirm, in particular, the metastability of the He 2+ 2 ground state, but exclude the possibility of a metastable 1 Sigma + g state for the recently experimentally observed Ne 2+ 2 .