We report a detailed ab initio study of the trapping behavior of interstitial helium atoms (IHAs) in hcp Ti.The tetrahedral interstitial site for one He is confirmed to be the most stable IHA configuration,but the most favorable interstitial site comprises two adjacent octahedral sites for two helium atoms.The octahedral IHA can trap another IHA regardless of the site where it is initially located,whereas the tetrahedral IHA cannot.Hybridization among the different states is responsible for the stable order,which has significant implications for He clustering and bubble nucleation that can affect material performance in future fusion reactors.These results provide the basis for the development of improved atomistic models.