Theoretical studies have been carried out on the structure of the endohedral CoGe10- cluster anion using ab initio (HF) and density functional theory (DFT-B3LYP) methods in conjunction with effective core potential basis sets (LanL2DZ and LanL2DZ*). For the important structures, all-electron basis sets plus polarization and diffuse functions (6-31+G*) are also used. At the levels of calculations employed, the bicapped tetragonal antiprism structure with D-4d symmetry is found to be most stable. Compared with the small fragments [Co + Ge-10(-)(D(4)d)], this structure is 97.7 kcal/mol more stable at the B3LYP/LanL2DZ level. Other low-energy structures have also been optimized on the potential energy surface of the endohedral CoGe10-cluster anion. (C) 2002 Published by Elsevier Science B.V.