We present a method (the Aufbaul Abbau method) for optimizing the structure of a whole series of clusters without making any assumptions on the structure. Subsequently, the method is combined with the embedded-atom method in determining the structure of the two energetically lowest isomers of Ni-N clusters with N up to 150. Finally, various analytical descriptors are introduced that are used in studying the overall shape of the clusters, their structure and stability, and possible growth and dissociation processes. (C) 2003 Elsevier Science B.V. All rights reserved.