ELECTRONIC STATES OF YN(N=2-4)

We compute the geometries and energy separations of several electronic states of Y(n) (n = 2-4). The complete-active-space self-consistent-field (CASSCF) followed by multireference singles+doubles configuration interaction (MRSDCI) calculations which included up to 2.6 million configurations are made in this study. We find two nearly degenerate states, namely, 3T1 and 1A1 of tetrahedral geometry, as candidates for the ground state of Y4. The Y-Y bond lengths are computed as 3.41 and 3.42 angstrom for 3T1 and 1A1 states, respectively. The electronic states with the rhombus structures are found to be greater-than-or-equal-to 0.34 eV above the tetrahedral ground state for Y4. We found two nearly degenerate electronic states with D3h geometries as candidates for the ground state of Y3 (2A2'' and 2A1'). Two electronic states Of Y2, namely, 5SIGMA(u)- and 1SIGMA(g)+ (short R) are also studied.