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

► How to define the microcanonical temperature of weakly bound clusters such the clusters of water molecules? ► Previous experience of different research groups has showed that the internal kinetic energy of such systems distributes non-uniformly between vibrational degrees of freedom. ► The method of effective modes is suggested to extract the collective modes of motion in the cluster which are active at different initial excitation energies and to calculate the microcanonical temperature with physically reasonable value.