The CCSD(T)/11e-RECP//MP2/11e-RECP method was used to explore the potential energy surfaces (PESs) of the formation of Ag-n (n = 2-6) clusters. Two kinds of reaction mechanisms were revealed in the formation of Ag-n clusters, the association mechanism for the formation of Ag-2, Ag-5, and Ag-6 clusters and the association-isomerization mechanism for the formation of Ag-3 and Ag-4 Clusters. Based on the canonical transition state theory, the calculated rate constants of the formation of Ag-n clusters displayed an odd-even effect: the rate constants of formation of Ag-n clusters with odd number were larger than those with even number. The rate constant of formation of Ag-4 was the lowest, whereas that of Ag-5 was the highest among Ag-n (n = 2-6) clusters. The formation of Ag-4 was the most difficult step in the aggregation process of the silver clusters. The formation of Ag-4 may be related with the critical point in the silver aggregation process. (c) 2006 Elsevier B.V. All rights reserved.