  The existence of intrinsic vacancies in cubic (monoclinic) TiO suggests
opportunity for hydrogen absorption, which was addressed in recent experiments.
In the present work, based on first principle calculations, the preferences are
studied for the hydrogen absorption sites and diffusion paths between them. The
oxygen vacancies are found to be primary hydrogen traps with absorption energy
of -2.87 eV. The plausible channels for hydrogen diffusion between adjacent
vacancy sites (ordered in the monoclinic TiO structure) are compared with the
help of calculations done with the nudge elastic band method. Several
competitive channels are identified, with barrier heights varying from 2.87 to
3.71 eV, that is high enough to ensure relative stability of trapped hydrogen
atoms at oxygen vacancy sites. Moreover, the possibility of adsorption of
molecular hydrogen was tested and found improbable, in the sense that the H2
molecules penetrating the TiO crystal are easily dissociated (and released
atoms tend to proceed towards oxygen vacancy sites). These results suggest that
hydrogen may persist in oxygen vacancy sites up to high enough temperatures.
