Using ab initio methods, a single isomer of D-3h symmetry was found to be preeminent on the Ca-3(PO4)(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(PO4)(2)](n) system with n being 2 or 3. This gives us an energy reference for analyzing the stability of the so-called Posner's cluster Cas(PO4)a 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.