The relative stability of water clusters of two different morphologies has been examined with the size N ranging from 54 to 864; the first one is a circular fragment of bilayer ice (a disk-like cluster), and the second one is a spherical fragment of normal cubic ice (a droplet-like cluster). We found a crossover at N-c similar or equal to 1000, below which the disk-like cluster becomes more energetically favorable. The crossover arises because the potential energy (per molecule) for the droplet-like cluster decreases linearly with N-1/3 whereas it decreases by N-1/2 for the disk-like cluster. (C) 1999 Elsevier Science B.V. All rights reserved.