Existence of Posner's cluster in vacuum

Using ab initio methods, a single isomer of D-3h symmetry was found to be p reeminent on the Ca-3(PO4)(2) cluster potential energy surface. This isomer has the distinct feature that each calcium atom is surrounded by four oxyg en atoms. We studied the aggregation of this isomer in the [Ca-3(PO4)(2)](n ) system with n being 2 or 3. This gives us an energy reference for analyzi ng the stability of the so-called Posner's cluster Cas(PO4)a which is the c ore of the actual structural model of amorphous calcium phosphate. Posner's cluster is shown to be significantly stabilized in comparison to the energ y reference.