Structure and energy separations of lowhyphen;lying electronic states of Tln(n=3ndash;5) clusters and their positive ions are computed using the completehyphen;activehyphen;space multiconfiguration selfhyphen;consistent field (MCSCF) (CASSCF) and the configuration interaction (CI) methods. The ground state of Tl3is4A2with a nearhyphen;equilateral geometry. Three nearhyphen;degenerate states (3B3u,1Ag, and1Au) are found as candidates for the ground state of Tl4with a perfect square geometry. The rhombus and the tetahedral geometries of Tl4are 5 and 9 kcal/mol higher in energy, respectively. In the case of the Tl5cluster, three important structures withC2v,C4v, andD3hsymmetries are considered in the present study and it is found that theC2vandC4vgeometries are the possible candidates for the groundhyphen;state geometry. The ionization energies of these clusters are also calculated. Mulliken population analyses and dissociation energies are given.
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