Crystallization of amorphous calcium carbonate (ACC) into one of the anhydrous crystalline polymorphs (vaterite, calcite, and aragonite) is a topic of extensive current research in biomineralization. We performed molecular dynamics simulations of ACC to characterize the dehydration-induced variation in the distance-dependent orientational order of Ca2+ cations around CO32- anions and to examine how such changes in the local structure facilitates nucleation in ACC. Our observations provide molecular scale basis for the formation of vaterite by internal reorganization of ACC nano-particles, which is in-line with the previous experimental demonstration that vaterite is the precursor for the crystallization of calcite or aragonite. (C) 2013 Elsevier B. V. All rights reserved.