Of Si <sub> 4 </ Sub> The O <sub> 4 </ Sub>, Of N-pentane And Sr <sub> Yruo <sub> 2 </ Sub> 6 </ Sub> First Principle

First Principle which is a method of calculation based on the principles of quantum mechanics is used widely in science, such as the structure, stability, electrical conductivity and spectral of material or clusters. And study of those properties of material and clusters is the most alive subject branch, with very abundant research connotation in the scientific and technological fields, and it is occupying central position in research application of the whole new material. Now the using of large and high-speed electronic computer make the advantage of the theory research more and more outstanding. The material experiments can be verified, interpreted, replenished and even predicted well by the theory. In this paper, the structure, the vibration frequency and spectrum of the silicon oxide tetramer (Si4O4) and the n-pentane under high pressure have been studied by use of the density function theory (DFT)B3LYP method, the conclusion is that there are two structures of Si4O4, and the phase change of n-pentane is verified with experiments. At the same time. First Principle is applied to study the structure and band of Sr₂YRuO₆. The material with order and characteristic of Mott insulators are analyzed basis on the calculation. The main contents are the following:1,The structure, vibrational frequencies, corresponding normal mode assignments, the infrared intensities and Raman activity of the silicon monoxide tetramer(Si₄O₄) in T_d symmetry and the isolated Si₄O₄ in D_2d symmetry are examined theoretically using the Gaussian03 set of quantum chemistry codes. All normal modes of buckled Si₄O₄ were 'Successfully obtained by a group theoretical analysis with the D_2d symmetry of the molecule. The conclusion is that Si₄O₄ possesses the cubane-like structure as C₄O₄ and Ge₄O₄. The planar Si₄O₄ may not be a stable structure. Comparing silicon monoxide tetramer with isolated buckled Si₄O₄, it shows that buckled Si₄O₄ can be observed easily because of it's lower energy. the two molecules are insulator and held together in part by electrostatic forces.2,The structure, vibrational frequencies, the infrared intensities and Raman activity of n - pentane are studied theoretically under high pressure whose range is from 0 to 17 GPa. The structural changes are consistent with the experimental data, but the changes at 12.3 GPa is strange. The Infrared and Raman spectra of n - pentane is also in agreement with the experimental results. The pressures for phase transition occurring in experiment and theory are nearly the same. Several analysis in intensity ratios according to the experimental conclusion is useful for predicting phase transition. We also conclude that: n-pentane prefers to be an in-all-trans conformation in the pressure range of 3-12.3 GPa, and has end gauche conformers above 12.3 GPa, which is important to understand the solid to solid phase transition.3,The energies of all the possible structures of the double perovskite Sr₂YRuO₆ were calculated using First - principles method. The conclusion is that the energy of the double perovskite structure is lowest. It is consistent with the experimental result given by the X-ray diffraction data. There is an orderly XRD peaks in the experiment. We also computed the band of the double perovskite. It shows that Sr₂YRuO₆ is a Mott insulator material, this is also consistent with the experimental result.