The possible structures of C~nSi (n=28,29) are studied by using ab initio method. Our studies show that C~2~9Si has a closed-cage structure with the silicon atom incorporated into the carbon network, and the most stable structure of C~2~9Si is related to the most stable strucrure of C~3~0 fullerene. C~2~9Si has lower stabilityand higher chemical activity than the C~3~0 fullerene.For C~2~8Si,there are two kindsof structures, namely,(Ⅰ)exohedral and (Ⅱ)hole-defect (the silicon atom is connected to four carbon atoms). Our calculated results show that,forC~2~8Si,the exohedral complexes are more stabl,i. e.,complexes(Ⅰ) are the main ingredient,and the most stable structure of C~2~8Si is formed by the most stable C~2~8 fullerene with an exohedral silicon atom. Our calculated results are in agreement with the experimental ones.

Key words: AB INITIO CALCULATION, CAGE STRUCTURE, SILICON, DOPE