| Summary: | 碩士 === 國立交通大學 === 應用化學系碩博士班 === 99 === For describing excited states and open-shell systems, multireference perturbation theory (MRPT) is the most reliable and computationally feasible among accurate electron-correlation methods. However, the MRPT methods are subject to a serious problem, known as the intruder state problem. The intruder states shall cause discontinuities in the potential energy surface of a system leading to non-physical results. The use of shift techniques in the MRPT may efficiently eliminate intruder states. Nevertheless, the chemical properties of the system studied using MRPT can change strongly depending on the parameter employed in the shift techniques. We therefore undertook an investigation for available shift techniques (e.g., real shift, imaginary shift, and intruder state avoidance techniques) to find appropriate values of these shift parameters. In this Thesis, 65 potential energy curves of low-lying electronic states of 15 diatomic molecules were studied using different MRPT methods combined with various shift techniques. The potential energy curves and their spectroscopic constants were critically evaluated for various values of the shift parameter, and compared with experiment. Finally, we used statistical analysis to determine the optimal value of the shift parameter in each shift technique. The research results show that the shift techniques can efficiently eliminate the intruder states and at same time they also reduce the error in spectroscopic parameters of the studied system when an optimal value of shift parameter is used.
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