Quantum Chemistry Calculations for Molecular Ground State Energy and Chemical Reactions

• Main Authors: Leu,Shyn-Yi, 呂世伊
• Other Authors: Prof. Mou,Chung-Yuan
• Format: Others
• Language: zh-TW
• Published: 1994
• Online Access: http://ndltd.ncl.edu.tw/handle/83041225861563722937

博士 === 國立臺灣大學 === 化學學系 === 82 === The thesis contains two topics, in the first part, a new calculational strategy was developed for molecular electronic system; and UMP4/6-31+G*//UHF/6-31+G* level calculations for reductive cleavage reactions were performed in the second part. For the calculations of electronic systems, in addition to the common-used ab-initio method, a new method, named Quantum Monte Carlo, using random-walks can also yield the exact solution of Schrodinger equation. Two fundamental obstacles are frequently met in the current Quantum Monte Carlo calculations, one is the finite step-size, and the other is the node problem. A new algorithm was developed to reduce these two problems. Floating Gaussian functions instead of Slater functions are used to be the guiding function. This leads to many harmonic velocity field generated. The Ornstein-Uhlenbeck process can exactly describe the random-walks in these velocity fields. Free from finite step- size error and avoiding the node in a novel way are the features of our method. Our new algorithm was applied to hydeogen molecule , LiH and HF molecules. Compared to other'' s works, the results were very good. The new algorithm using Floating Gaussian functions as guiding function with the Ornstein-Uhlenbeck process has opened a new way in Quantum Monte Carlo calculation. MP4 level caulculations are used to study the reductive bond cleavage reactions. The reaction is a concerted electron transfer -bond breaking process in accord with previous experimental findings. The equilibrium geometry and bond dissociation energy of C-X bond thus found are in good agreement with the previous theoretical and experimental results. The anomeric effect and electrostatic effect are used to describe the differences in charge separation and bond energy and bond length of C-X bond between CH3X and CH2X2.