| Semiconductor cluster have attracted much attention over the last decade due to their unique physical properties and potential use in a wide range of applications. Recently, high-quality II～VI semiconductor (CdSe and HgTe) quantum dots(QDs) synthesized by colloidal chemistry have emerged as attractive candidates for use in light emitting devices with tunable emission. In many previous theoretical studies, the semiempirical methods have been applied to investigate properties QDs. However, there have been much fewer first principles studies due to the computational demands. Therefore, it is significant to further study the structures and electronic properties of QDs and clusters by using first principles. In this paper, using the first principles method which base on density function theory(DFT), we calculated the structures and electronic properties of ground states of (CdSe)n and (HgTe)n(n=17) clusters with the generalized gradient approximation(GGA). To compare the results, we also carried out the calculations within the local density approximation (LDA). We come to the following conclusions: (1) Our results of (CdSe)n(n=1-7) are in good agreement with those of M.C.Troparevsky et al.. This shows that our calculation scheme is reasonable. (2) The results show that high symmetry arrangements occur in geometrical structures of the ground state for the (CdSe)n (n=1-6) clusters. However, for n=7, the symmetry is much lower. (HgTe)n and (CdSe)n exhibit similar structural symmetries. However, there are small discrepancies in both the length of bonds and bond angles between them, which lead to the difference of the material (i.e., CdSe crystal structure is zincblende and HgTe is wurtzite). (3) There are three different structures near the ground state of (CdSe)4 (4A, 4B and 4C). Structure 4C is more stable, and the binding energy of which is 7.10% higher than that of 4A and 3.54% higher than that of 4B. There are two different structures around the ground state of (CdSe)5(5A and 5B). Structure 5A is more stabe, and the binding energy of which is 1.27% higher than that of 5B. Accordingly, there are three different structures near the ground state of (HgTe)4 (4A, 4B and 4C), and structure 4C is also more stable, while the binding energy of which is 3.95% higher than that of 4A and 15.26% higher than that of 4B. For (HgTe)5 cluster, structure 5B is more stable, and the binding energy of which is 6.19% higher than that of 5A. The relation between (CdSe)7 and (HgTe)7 is similar with (CdSe)5 and (HgTe)5. There are two different structures around the ground state both (CdSe)7 and (HgTe)7 (7A and 7B), and structure 7B is more stable . Therefor, we come to conclusion that the binding energy of clusters depends not only on the symmetry, but also lead to the difference of the material. (4) (CdSe)n(n=1-7) clusters with n=4 have substantially higher energy gaps and those with n=5 have lower ones. The variation in the energy gaps as a function of cluster size for (HgTe)n clusters exhibits different trends with that for (CdSe)n clusters besides that peak occurs with n=4 and without lower ones in the size which we study on, which indicates that different quantum-size effects occur between the two different material clusters. As for correlation between binding energy and energy gaps, results within GGA exhibit more clearly than those within LDA. We have investigated (CdSe)n and (HgTe)n clusters with n ranging just from 1 to 7 due to the computational demands. In order to gain full information of II-VI semiconductor (semimetal), it is necessary to investigate larger size and the other semiconductor (semimetal) clusters such as (CdTe)n and (HgSe)n, which is what we need to do for further investigation.
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