| Atomic and molecular clusters (clusters) are relatively stable micro or sub– micro aggregates which are composed of several to more than thousands of atoms or molecules (the atomic number is considered in the range of 10-105 by the majority of the international communities) by some physical or chemical bonding forces[1-2]. The special physical and chemical properties and size-dependence of clusters arouse lots of attentions in the physical, chemical and materials fields. The basic problem of the science of clusters is to research that how do the clusters of atoms or molecules evolve, as well as the changes of structures and properties of the clusters with the evolutions, and which size of clusters can transite into a macroscopic solid. To study on the natures of clusters adequately, the ground state structures of various types of clusters should be derived first. However, it is very difficult to find cluster structures, so this problem is called as np-hard problem. At present a lot of ways have been found to study the structures of clusters, such as hop basen algorithm, genetic algorithm, empirical potential aclgorithm, etc.As we known, the hydrogen bonds widely existe in varies substances, and as the interaction force between molecules they play an important role in many scientific issues. Many problems can be solved by the research of the hydrogen bonds, such as energy transfer, dissolution and chemical reactions and so on. It can do a solid foundation for the development of water resources in nature. As two kinds of special and important hydrogen bonds: O…H and N…H existe in ammonia clusters. Exploring the stable structures of the ammonia clusters is rational theory groundwork for researching the components of atmosphere and the formation of biological macromolecules. Furthermore, ammonia is a typical soluble polar solvent, so the research on the ammonia will have a great practical significance. In this article, we first use the empirical method to obtain the initial structure of the ammonia clusters, and then use DFT (DFT/B3LYP) and the two perturbation methods (Moller-Plesset/MP2) for the geometry optimization and frequency calculations of (NH3)2(H2O)3 cluster, and (NH3)2(H2O)6 cluster at the 6 -31G (d, p) and the 6-311 + + G (d, p) basis set level. Finally 15 classes of (NH3)2(H2O)3 cluster stable strctures, and 12 classes of (NH3)2(H2O)6 clusters stable structures are derived in the level of the B3LYP/6-311 + + G (d, p). The study shows that the most stablest conformation for the (NH3)2(H2O)3 cluster is a positive pentagon ring structure, and the most stablest strcture for the (NH3)2(H2O)6 cluster is octahedral cage-like structure. The interaction between water molecules is the strongest, and the interaction between ammonia molecules is the weakest among the forces between molecules. Based on the frequency analysis, we found that the type of hydrogen bonds have an important effect on the size of the frequency shift, which is consistent with the conclusion we derived before, and the size of dipole moment and structural stability shows no necessary relations.The main contents of this dissertation are shown as follows:In chapter 1, firstly we simply introduce the concept and research background,classification,the main character of cluster. Secondly, we presented the main research directions and difficult problems of cluster in nowadays. Lastly, we show the good future of cluster research and our work.In chapter 2, we introduced some theory methods of clusters studies, including variational principle and perturbation theory, density functional theory, the basis functions of self-consistent calculation, the calculation method of vibrational frequencies and a brief description of the Gaussian 03 program.In chapter3, The structures and Infrared Spectra of (NH3)2(H2O)3 clusters were studied using density functional theory methods (DFT/B3LYP) and M?ller-Plesset second-order perturbation method (MP2) with the 6-31G(d) and 6-311++G(d, p) basis sets. We found 15 stable geometries of (NH3)2(H2O)3 clusters, The result indicates that the pentagon structure is the most stable structure of the The result indicates that the pentagon structure is the most stable structure of the (NH3)2(H2O)3 clusters.In chapter 4, we investigated the geometries, stabilities of (NH3)2(H2O)6 water-ammonia clusters. We found that the most stable configuration of (NH3)2(H2O)6 clusters is regular octahedron cage-like structure. In chapter 5,we have a summary and outlook of our work.
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