Eight isomers (planar hexagon, benzvalene, Dewar benzene, triangular prismatic, bicyclopropenyl, octahedron, chair form, and twist boat form) of Si-6, Si-6(2-), Si-6(4-), and Si-6(6-), have been searched for by the MP2 and B3LYP electronic structure calculations. Totally 14 isomers were found: two Si-6, six Si-6(2-), five Si-6(4-) and one Si-6(6-). Two of them are different from the eight isomers: deformed triangle Si-6(2-) pentagonal pyramida Si-6(4-). We discovered that the predicted stable shapes of Si-6(2-), Si-6(4-) and Si-6(6-) are octahedral, pentagonal pyramidal, and hexagonal, respectively, which agrees well with Wade rule. (c) 2005 Elsevier B.V. All rights reserved.