| This work presents the study of quantum dynamics in realistic chemical systems through the use of various theoretical and computational methods. We apply these methods to model the quantum transport of molecular H{dollar}sb2{dollar} in ice and water which can be represented in terms of a few quantal variables coupled to a classical bath of many degrees of freedom. This chemical system has numerous experimental results available which affords the unique opportunity for detailed comparison between theory and experiment. Therefore, we can use this realistic system to test the reliability of approximations made by each method.; The transport of H{dollar}sb2{dollar} in ice and water can be handled using various levels of approximation, including classical, adiabatic and nonadiabatic dynamics. Since tunnelling probably plays a significant role in the mechanism of H{dollar}sb2{dollar} transport, the quantum treatment of the nuclear translational motion is necessary. This transport can occur adiabatically on the ground state, or through excited states where motion of the classical H{dollar}sb2{lcub}rm O{rcub}{dollar} subsystem drives nonadiabatic transitions between the states of the quantal H{dollar}sb2{dollar} subsystem. We use new general multidimensional methods in our calculations.; An alternative, approximate formulation of quantum dynamics is the approach of Cao and Voth's centroid dynamics in which path integral methods are used to represent a thermal distribution of quantum states known as the centroid density. The path centroid is treated as a classical particle which moves over an effective potential approximately incorporating quantum effects. Centroid time correlation functions are obtained which relate to real time quantum correlation functions via Kubo transformation.; We explore the benefits and drawbacks of these different methods for various state points for H{dollar}sb2{dollar} in H{dollar}sb2{lcub}rm O{rcub}.{dollar} We calculate diffusion coefficients for H{dollar}sb2{dollar} in ice and water and the rotational Raman spectrum for H{dollar}sb2{dollar} in ice. We also recalculate the spectra in water with quantum treatment of the H{dollar}sb2{dollar} translational motion and compare to previous calculations of Xiao and Coker where H{dollar}sb2{dollar} translational motion in water was treated classically. We compare our results with Strauss' experimental measurements of H{dollar}sb2{dollar} diffusion in ice and H{dollar}sb2{dollar} rotational Raman spectrum in ice and water. |
