| Summary: | 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.
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