Ab initio electronic structure calculations using a triple-' plus polarization basis set, second-order perturbation theory, and coupled cluster theory show the dimerization of Ti& to be kinetically and thermodynamically very favorable. Six minima have been found on the potential energy surface of ThH8 : two with double hydrogen bridges and four with triple hydrogen bridges. This potential energy surface is very flat suggesting rapid interconversion between these isomers is possible. The large thermodynamic driving force for dimerization (up to -46.1 kcal/mol on the classical surface) is attributed to both electrostatic effects and the electron deficiency of titanium.