Ab initio calculations for electron attachment to Cl-2

Ab initio calculations are performed for the two species involved in the at tachment of an electron to Cl-2: the neutral ground state ((1)Sigma(g)(+)) and the four anionic states ((2)Sigma(u)(+), (2)Pi(g), (2)Pi(u), (2)Sigma(g )(+)). Four approaches are used: CASPT2, CASPT3, AQCC and CCSD(T). The good agreement with the known spectroscopic constants for Cl-2 gives confidence in the present results. The neutral-anionic crossings have been carefully located. For the most reliable method, CCSD(T),we found the crossings at 95 , 441, 955 and 1276 meV above the neutral ground vibrational level, for (2) Sigma(u)(+) (2)Pi(g), (2)Pi(u) and (2)Sigma(g)(+) anionic curves respective ly The recently observed resonance around 100 meV in electron scattering is thus assigned to the lowest crossing only, in contradiction with the previ ously suggested mechanism which also involved the Pi states of Cl-2(-).