Existence of Posner's Cluster in Vacuum

Using ab initio methods, a single isomer of D 3 h symmetry was found to be preeminent on the Ca 3 (PO 4 ) 2 cluster potential energy surface. This isomer has the distinct feature that each calcium atom is surrounded by four oxygen atoms. We studied the aggregation of this isomer in the [Ca 3 (PO 4 ) 2 ] n systemwith n being 2 or 3. This gives us an energy reference for analyzing the stability of the so-called Posner's cluster Ca 9 (PO 4 ) 6 which is the core of the actual structural model of amorphous calcium phosphate. Posner's cluster is shown to be significantly stabilized in comparison to the energy reference.