Abstract: The equilibrium geometries and electronic properties of Cu$_{m}$Si$_{n}$ ($2 \le m+n \le 7$) clusters have been studied by using density functional theory at the B3LYP/6-311+G (d) level. Our results indicate that the structure of CuSi$_{n}$ ($n<$6) keeps the frame of the corresponding Si$_{n}$ cluster unchanged, while for Cu$_{n}$Si clusters, the rectangular pyramid structure of Cu$_{4}$Si is shown to be a building block in many structures of larger Cu$_{n}$Si clusters. The growth patterns of Cu$_{m}$Si$_{n}$ clusters become more complicated as the number of Cu atoms increases. Both the binding energies and the fragmentation energies indicate that the Si--Si bond is stronger than the Cu--Si bond, and the latter is stronger than the Cu--Cu bond. Combining the fragmentation energies in the process Cu$_{m}$Si$_{n} \to $Cu+Cu$_{m - 1}$Si$_{n}$ and the second- rder difference $\Delta_{2}E$($m)$ against the number of Cu atoms of Cu$_{m}$Si$_{n}$, we conclude that Cu$_{m}$Si$_{n}$ clusters with even number of Cu atoms have higher stabilities than those with odd $m$. According to frontier orbital analyses, there exists a mixed ionic and covalent bonding picture between Cu and Si atoms, and the Cu d orbitals contribute little to the Cu--Si bonding. For a certain cluster size ($m+n$ = 3, 4, 5, 6, 7), the energy gaps of the most stable Cu$_{m}$Si$_{n}$ clusters show odd--even oscillation with changing $m$, the clusters with odd $m$ exhibit stronger chemical reactivity than those with even $m$. 

Key words: Cu_mSi_n clusters, density functional theory, structures and properties