On the definition of the microcanonical temperature of small weakly bound molecular clusters

The concept of the microcanonical temperature for weakly bound clusters is re-examined. A parameter defined as the number of effective modes is suggested as a measure of complexity of distribution of the internal energy between the collective modes of a cluster. The water pentamer is considered as an example. The temperature regions for solid-like and liquid-like phases are obtained and the typical collective motions of the water molecules are estimated for each phase.