Development And Application Of The Parallel Tempering Method

Nucleation from solution carries broad implications. The study of nucleation processes has been a driving force for technological advances and our understanding of the nature. Due to the extensive time and space scales involved, the mechanism of nucleation is far from clear. Experimental techniques cannot capture the detailed dynamics of chemical reactions, whereas computer simulation is a promising tool to investigate microscopic phenomena. Nevertheless, direct modeling of reactions under mild conditions remains a grand challenge: sampling of rare events like chemical reactions is computationally expensive, especially when using accurate first-principles models.Thus there is urgent need for development of efficient models and sampling methods.This thesis considers the nucleation of silica in the context of zeolite synthesis. With the help of high performance computing resources, the initial reactions in zeolite synthesis were simulated by using ReaxFF reactive force field and replica-exchange molecular dynamics. The results show that inorganic structure-directing agents promote the aggregation of silicate, the formation of branched and cage structures, and temperature influences the interaction strength between structure-directing agents and silicate, coordination number and ring size distribution. The thesis also covers theoretical study of sampling methods. Through asymptotic variance analysis, it was demonstrated that the efficiency of parallel tempering was determined by both exchange attempt frequency and the distribution of the extended ensemble. Then, by examining the properties of parallel tempering and simulated tempering at the infinite swapping limits, the formal relationship between these two methods and the multicanonical algorithm was established. This relationship accounts for the relative inefficiency of parallel tempering and suggests a unified way to optimize the generalized ensemble methods. The attempt and methodology development presented here are expected to be helpful for future theoretical study of chemical reactions.