A Quantum Monte Carlo Study of Molecular Titanium Dihydride$^\dag$

Recently there has been interest in the possibility of reversibly storing molecular hydrogen on titanium decorated carbon-nanostructures$^1$. As part of our research$^2$ in this area, we present DFT and QMC results for molecular TiH$_2$ using pseudopotentals. We identify the low energy symmetry-classified states and compare with previous work$^{3,4}$, where there is not a consensus on the symmetry and geometry of the ground state. At the DFT level, the TiH$_2$ d-states are nearly decoupled from the molecular geometry so that several d-state orderings are very close in energy. In our work we use diffusion Monte Carlo with the fixed-node approximation where the symmetry and nodal structure are determined by a trial function constructed of molecular orbitals from DFT. We will also discuss progress on Ti-carbon systems pertaining to hydrogen adsorption. \\ \\ $^1$ E. Durgun \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 226102 (2006). \\ $^2$ T. D. Beaudet \textit{et al.}, J. Chem. Phys. \textbf{129}, 164711 (2008). \\ $^3$ J. A. Platts, J. Mol. Struct. \textbf{545}, 111 (2001). \\ $^4$ B. Ma, C. L. Collins, H. F. Schaefer, J. Am. Chem. Soc. \textbf{118}, 870 (1996). \\ $^\dag$ Supported by NSF DMR03-25939.