中文摘要：

使用价键优选法，在较小的计算量下，系统地得到了二、三周期元素团簇体系（直到四原子为止）关键结构的性质．还能得到团簇从双原子到四原子的演化过程．对所有团簇的电子结构进行分析，可以理解特定元素的四原子团簇出现Td立体结构的原因．更加有趣的是，通过较小团簇结合能和对应元素单质宏观性质（熔点、沸点等）的相应变化关系，能够理解在熔解、沸腾过程中关键结构的作用．

英文摘要：

Using optimum valence bond scheme which reduces the computation effort, we study systematically the properties of the Critical structures （critical points） of small clusters （up to four atoms） of elements in the second and third row of the periodic table. We also show the evolution process of the clusters from two atoms to four atoms. By examining the electronic structures of all clusters, we can understand why the four-atom clusters for specific elements can have three-dimensional structures with Td symmetry. More interestingly, comparing the variation of the binding energy of such small clusters with the melting points and boiling points of corresponding pure element materials, we can understand the effect of the critical structures in the melting and boiling processes.