Understanding the geometry structures of gold clusters, especially with adsorbates, is essential for designing highly active gold nanocatalysts. Here, we report a detailed theoretical study of the geometry structures of bare and CO-saturated Au(n)(+) (n = 4-6) clusters. It is found that the chemisorption of CO molecules leads to significant geometry changes of the gold clusters from two- to three-dimensions (3D), even for clusters as small as Au(4)(+). These gold cationic clusters exhibit characteristic coordination binding sites that have distinct electronic structures. We also find that commonly used density functional theory (DFT) methods have difficulty in accurately predicting energies of some isomers of Au(n)(+) clusters or Au(n)(CO)(n)(+) complexes, with the calculated relative energies strongly depending on the exchange-correlation functionals used. Caution must be exercised when using DFT methods as a blackbox for predicting the structures and energies of gold clusters.