| It is more and more exact to describe small molecules with the developing and improving of experimental techniques and instruments;correspondingly, the desire has become higher and higher for some theoretical work such as excited states, chemical reaction and potential energy surface (PES) etc. A precise and effective program is the assurance for high level calculation. In this dissertation, several kinds of PESs were constructed by using the precise quantum chemistry program Xi'an-CI written by our group;besides the program was modified to expand and perfect its function.This dissertation consists of three chapters. Chapter 1 presents the theoretical background of this thesis. Firstly, we introduct the most popular and well-developed methods that are used to calculate the electronic correlation energy, which were all used in the thesis. Some multi-reference methods such as MCSCF, MRCI, DCCI and MRPT2 are discussed as the main subjects. Secondly, the theoretical basis of constructing the PES is presented, including the calculation, fitting, and the crossing of PES.In the following two chapters, some typical molecular systems are calculated using our Xi'an-CI program. In chapter 2, the potential energy curves for three states of C2 and the potential energy curve for the ground state of N2 are constructed by using many multi-reference methods such as CASSCF, MRCI, DCCI and MRPT2. In order to review the advantage and disadvantage of each method, these curves are benchmarked against the most accurate theoretical method FCI and experimental spectroscopic parameters, respectively. In chapter 3, the potential energy surfaces for the nine low-lying electronic states of SO2 have been constructed by using MRPT2 with the basis set cc-pVTZ. The optimized geometries and the adiabatic excitation energies of these states are in good agreement with experiments and previous calculations. The crossings and avoided crossings displayed in the potential energy surfaces are expounded. |
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