Global minima of clusters Cn-xSix (n = 9, 10, 11, x = 0-n) were investigated by a combination of molecular mechanics (MM) and quantum mechanics (QM) methods. The results showed that when the number of C atoms is large enough, C atoms prefer to form pentagons.. hexagons, or even large carbon rings. The rest C and Si atoms form triangles and quadrangles around the carbon rings. When the number of Si atoms increases, the structures obtained with MM method distort dramatically when they are further optimized with QM method. The reason might be the inaccuracy of Brenner potential in describing Si atoms. (c) 2005 Elsevier B.V. All rights reserved.