Economic basis sets for ab initio calculations of systems involving titanium (Ti) atom have been studied. The basis set in which the polarization and diffuse functions are applied only to Ti and its nearest neighboring atoms is shown to give reliable binding energies and geometries for various carbon-based compounds. Such a basis set is used as the economic basis set for calculations of medium-sized clusters simulating, Ti and diamond interactions. For large clusters composed of Ti and C atoms, the more economic basis set 6-31G is considered for obtaining reliable energetics with the correction using high-level single-point energy calculations with 6-31+G*. The approach to determine the economic basis set is expected to be useful for a general system containing transition metal element. With the economic basis sets, simulations of diamond metallization have revealed reasonable behavior of the Ti atomic interaction with diamond film. On the basis of the determined energetics of Ti deposition, the high Ti concentration in CVD diamond after metallization is conjectured to occupy the grain boundaries rather than the bulk of diamond grain. For a Ti atom with high kinetic energy, the Ti atom may penetrate through a diamond (001) surface more easily than through a diamond (I 11) surface. In the diamond bulk, the Ti atom may energetically favor to substitute a C site more than diffusion.