ELECTRONIC EXCITATION-ENERGIES IN COPPER SELENIDE CLUSTERS

We report calculated singlet and triplet excitation energies in bare c opper selenide clusters (Cu2Se)(n) up to n = 18 and ligand-covered clu sters (Cu2Se)(n)(PH3)(m) up to n = 10 (m less than or equal to 8). Var ious approaches for the calculation of excitation energies (including coupled cluster methods) are compared for small clusters which suggest s the method of configuration interaction restricted to single substit utions (CIS) to give reasonable results with an error in the predicted energies of roughly +1 eV. The lowest CIS excitation energies show a steep increase with the cluster size up to about n = 6 and a slow decr ease for larger clusters, for which a rationalization is given. Even f or the largest of the treated clusters the computed excitation energie s are higher than 3.5 eV which suggests that these clusters are still far from showing beta Cu2Se bulk properties (energy gap 1 eV). The ele ctronic excitations are found to be essentially of charge transfer cha racter (Se-->Cu). (C) 1996 American Institute of Physics.