The NVT ensemble of water octamer is divided into the configurational subsets, which correspond to the topology-distinct H-bond patterns, and the relative molar Helmholtz energies of the H-bond patterns are evaluated. The method is based on the combination of standard Monte Carlo techniques with defined H-bond patterns. The structure distributions of water octamers at 200 and 300 K are presented based on the H-bond patterns instead of the `inherent structures'. The thermodynamically favored structures of water octamer, which are energetically favored and readily feasible (entropy-favored for cluster formation), are presented. (c) 2006 Elsevier B.V. All rights reserved.