| Abstract: Atomic and molecular clusters are relatively stable micro or sub-micro aggregates which are composed of several to more than thousands of atoms or molecules by physical or chemical bonding forces. The special physical and chemical properties and size-dependence of clusters have aroused some people to pay attentions in the field. The properties of clusters changes with the changing number of the atoms or molecules contained in the clusters. The size of clusters ranges from one to tens of nanometer. The cluster has the special physical and chemical characteristics that distinct from a single atom and also different from molecular and liquid. So people regard it as new material level. The properties of the clusters are decided by its structure. So, the study on the structures of the clusters is a meaningful work. However, to find the structure of the cluster is a very difficult thing. Through the people constantly research, some people have developed a series of algorithms for solve it.The concept of hydrogen bond was presented in 1922. As a special interaction, it has being attracted the attention of scientists. Most of the researches on hydrogen bond are mainly focused on N? ? ?H and O? ? ?H. The main reason is that hydrogen bonds play an important role in the dissolution, energy transfer, and chemical reaction and so on. Because the ammonia and water clusters contain a lot of hydrogen bonds, researching the cluster has the profound meaning. First, we obtain the initial structures of (NH3)2(H2O)4 and [(NH3)2(H2O)m=2,3]- by using the empirical potential method. The structures of the (NH3)2(H2O)4 and [(NH3)2(H2O)m=2,3]- are then investigated by density function theory and M?ller-Plesset second-order perturbation methods with the 6-31G(d,p) , and 6-311++G(d, p) basis sets, respectively. After getting the stable configurations, the energy, hydrogen bond length, dipole moment, and other relevant properties are calculated and analyzed.The main contents of this paper are shown as follows:In chapter 1: We simply introduce the concept of cluster, the background of this research, classification, the main direction of the cluster research, the difficulty at this stage and the work of the author.In chapter 2: We introduce some theory and methods about the 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)4 clusters are studied. We obtain 16 stable geometries of (NH3)2(H2O)4 clusters. On the basis of the stable structure we calculate the energy and the average length of hydrogen bond, and further analyze the infrared vibrational spectrums.In chapter 4: The structures and Infrared Spectra of [(NH3)2(H2O)m=2,3]- clusters are studied using density functional theory methods (DFT/B3LYP) and M?ller-Plesset second-order perturbation method (MP2) with the 6-31G(d,p) and 6-311++G(d, p) basis sets. 10 stable geometries of the [(NH3)2(H2O)m=2,3]- clusters have been found. On the basis of the stable structure we calculated the energy, the average length of hydrogen bond, and further analyzed the infrared vibrational spectrums.In chapter 5: We present a summary and outlook of our work.
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