| Clusters as a micro-level, limited aggregation of atoms, are a special form between the gaseous and condensed form, cluster have a strange geometric structure. Clusters stability of the structure of clusters not only in-depth understanding of the nature, and also helps further understanding of the corresponding macro block of physical and chemical properties. Research clusters in theory, the first important thing is to identify clusters of ground-state geometry. There are many clusters of isomers, and the number of isomers with the cluster size increases and rapidly growing. In recent years, as people gradually in-depth research clusters, molecular interactions between the weak researchers have also been more and more attention.Combined with a weak hydrogen bonding interaction between the molecular clusters have become the theoretical and experimental study. NH3 molecule clusters, H2O molecule clusters and mixed clusters are typical cluster system. The ammonia water in the chemical cluster and the environment play an important role in transforming the field, at present, they have gradually attracted many researchers eyes.Since 1922 the concept of the proposed hydrogen bonding, research workers will be mainly concentrated in the research O - H and N - H hydrogen-bonded clusters of bond issues. As we all know, water and ammonia are easy to form hydrogen bonds. It is precisely because water, ammonia these characteristics, making water and ammonia small clusters of more and more widely. Beu and Buck paired with additional model potential calculate the geometric structure, bond energy and the vibration frequency of small ammonia clusters, Maheshwary, who calculated several possible small water clusters geometry by ab initio methods. Clusters of complex experiments from the perspective of a microwave and infrared spectra as well as from a theoretical perspective by ab initio calculation methods have been confirmed.In this article, we first got many geometry structures of the (H2O)(NH3)2 cluster and (H2O)(NH3)3 cluster by experience potential. Then we calculated the cluster geometry structures by quantum mechanics method. Group set up in the add-polarization and diffusion function, we have to use HF/6-31 + G (d, p) optimized geometry, as Density Functional Theory in B3YLP and MP2 HF theory on the basis of taking into account exchange relevant role Therefore, we calculated the geometry optimization, energy, frequency analysis using 6-311 + + G (d, p) basis set in the High Density Functional Theory B3YLP and MP2 level, by comparing the two the calculated results, finally the geometry of the (H2O) n (NH3) m (m≤5) cluster have been found. All calculation are used by Gaussian03 procedures.The main contents are:In chapter 1, we presented the definition of Molecular clusters, explained their properties and sorts, classified them and mentioned some difficult problems in study nowadays, as well as saw a good future.In chapter 2, we introduced some theory methods of clusters studies: the ab innito theory, the molecule orbital theory, the density functions theory, and electron correlation methods.In chapter 3, we given a brief description of the choice of potential and basis set(including slater type orbital and gauss type orbital),as well as the calculation of vibration frequency.In chapter 4, we investigated the geometries, stabilities of water-ammonia clusters. Further more, we have compared the energy order of water-ammonia clusters by three theories.
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