In this work, a strong correlation between the density of defected atom pairs at the crystal-liquid interface and the activation energy for crystal growth from deeply undercooled melts is established. Using molecular dynamics simulations we study the intermediate interstitial pairs at the crystal-melt interfaces of both BCC and FCC pure metals. It is found that the pairs have preferred orientations, and two important types of interstitial pairs at the interfaces are identified according to their orientations. We demonstrate that the relative amount of these two kinds of interstitial pairs varies greatly for different systems and dominates the activation energy for crystal growth. The dependence of local structure and energy on the orientation of the interfacial interstitial pairs are revealed and the connections of these features with crystal growth kinetics are disclosed.