Electronic structure calculations on the C4 cluster.

The ground and the electronically excited states of the C4 radical are studied using interaction configuration methods and large basis sets. Apart from the known isomers [l-C4(X(3)Sigmag (-)) and r-C4(X(1)Ag)], it is found that the ground singlet surface has two other stationary points: s-C4(X(1)Ag) and d-C4(X(1)A1). The d-C4 form is the third isomer of this cluster. The isomerization pathways from one form to the other show that deep potential wells are separating each minimum. Multireference configuration interaction studies of the electronic excited states reveal a high density of electronic states of these species in the 0-2 eV energy ranges. The high rovibrational levels of l-C4((3)Sigmau (-)) undergo predissociation processes via spin-orbit interactions with the neighboring (5)Sigmag + state.