Hierarchical approach to study water cluster in first-principles calculations
Nguyen, Quoc Chinh
Date of Issue2010
School of Physical and Mathematical Sciences
First-principles calculation plays a central role in computational physics and chemistry in studying the properties of molecular systems. Nevertheless, its high computational cost limits the capability to carry out first-principles calculations for exploring the potential energy surface of molecular systems. In this work, a hierarchical method has been proposed to handle this issue in which we combine the semi-empirical methods and first-principles calculation in order to reduce the computational demand. The proposed hierarchical method has been applied to study water clusters. The microscopic structures and properties of water clusters have attracted interest of theoreticians because they are extremely difficult to be revealed in experiments. In our studies, the potential energy landscape of protonated (H + (H2O)n ), deprotonated (OH – (H2O)n ) and neutral ((H2O)n ) water clusters were thoroughly explored at quantum chemistry level. The distinct configurational isomers of different kinds of water clusters were uncovered and archived systematically by using a so-called archival memetic algorithm. The optimized geometries and relative stabilities of each system were analyzed afterward. For studies on thermodynamics and structural transitions, harmonic superposition approximation has been used to investigate the thermodynamics at both empirical and first-principles levels. The accuracy of harmonic superposition approximation has been tested by comparing the results with the ones predicted by Monte Carlo simulations. The finite temperature effects, the structural transitions as well as the thermodynamic profile of each kind of water for different sizes have been investigated systematically. For bridging the gap between theory and experiment as well as testing the accuracy of the simulation results, the vibrational spectra were simulated based on the calculated thermodynamic properties. The vibrational spectra were subsequently compared with recent experimental results. In addition, the effects of zero-point energy correction on the relative stabilities, thermodynamic properties and vibrational spectra of each system were discussed throughout the studies. Last but not least, we reported the details on the development of the potential models for protonated hydrogen fluoride and deprotonated water clusters in order to extend the current work to other molecular systems and also to improve the efficiency of the hierarchical approach for our future studies.