A full-dimensional quantum Monte Carlo study of H5O2+
Full dimensional (15 degree-of-freedom) quantum calculations of vibrational energies of H5O2+ are reported using the global potential energy surface (OSS3(p)) of Ojamae et al. [L. Ojamae, I. Shavitt, and S. J. Singer, J. Chem. Phys. 109, 5547 (1998)]. As the smallest aqueous proton transfer system, the protonated water dimer, H5O2+, is an interesting case for vibrational study. The major difficulty in understanding this system lies in its high dimensionality and and strong coupling between anharmonic low frequency coordinates, among them motion of the central proton, which is associated with the most intense vibrational modes. The potential energy surface has been extensively studied theoretically, and some reduced dimensionality calculations of the vibrational structure have recently appeared. Full-dimensional calculations, however, are needed to benchmark the more approximate treatments of this molecule, and to provide a guide for the couplings that might be safely neglected. In our study, the diffusion Monte Carlo method is employed for a full-dimensional treatment of the system. An improved trial wave function which we have recently developed produces a highly accurate zero-point energy and ground-state wave function which gives better starting point for understanding the excited states. The quantum character of the central proton and the degree of coupling between several angular coordinates, torsions, and the central proton motion are discussed. The flexible trial function also allows us to calculate the energies of certain excited vibrational levels using the method of Ceperley and Bernu. Because of the complex nature of the eigenvalue spectrum it is not feasible to completely resolve the spectrum. Transition moments are calculated to help identification of the excited states.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:h5o2 protonated water dimer qunatum monte carlo correlation function quantum cfqmc excited states
Date of Publication:01/01/2004