STRUCTURES IN THE LONG-RANGE POTENTIAL CURVES OF THE NA2 MOLECULE - COMPARISON BETWEEN AB-INITIO AND ASYMPTOTIC CALCULATIONS

We have performed model potential and pseudo-potential ab initio calculations for the excited states of the Na2 molecule over a wide range of internuclear distances. We compare these results with long range calculations using a multipolar expansion for the Coulomb interaction and asymptotic estimation of the exchange term. For the first excited molecular states, the results of the two methods are in good agreement for R > 15 au. For the Rydberg states, some potential curves display structures due to pseudo-crossings between covalent curves (correlated to Na[nl] + Na[n'l'] dissociation limits) and ionic curves (dissociating into Na+ + Na-, the negative ion being in a stable or in an autoionizing state). For instance, we find a well of 2800 cm-1 at R = 30 au in the 61SIGMA(g)+ curve correlated to Na[3s] + Na[5s]. These structures can be confirmed in case of 1SIGMA(g)+ symmetry by long range calculations including the interaction between the covalent states and an ionic state.