  We report the prediction of temperature-dependent diffusion coefficients of
interstitial hydrogen, deuterium, and tritium atoms in $\alpha$-Ti using
transition state theory. The microscopic parameters in the pre-factor and
activation energy of the impurity diffusion coefficients are obtained from
first-principles total energy and phonon calculations including the full
coupling between the vibrational modes of the diffusing atom with the host
lattice. The dual occupancy case of impurity atom in the hcp matrix are
considered, and four diffusion paths are combined to obtain the final diffusion
coefficients. The calculated diffusion parameters show good agreement with
experiments. Our numerical results indicate that the diffusions of deuterium
and tritium atoms are slower than that of the hydrogen atom at temperatures
above 425 K and 390 K, respectively.
