Dodecahedral clathrate structures and magic numbers in alkali cation microhydration clusters.

Using global geometry optimization based on our specialized version of Genetic Algorithms, we have examined the global and most important local minimum energy structures of water microsolvation clusters of potassium and cesium cations within the common TIP4P/OPLS model. Together with our earlier results on the corresponding sodium case, this work constitutes a first step towards a theoretical elucidation of "magic numbers" of solvating molecules and proposed special structures occurring in these systems. In particular, the actual role of dodecahedral cage structures is examined. Within the present model, they do not occur in sodium microsolvation, in agreement with the absence of the magic number 20 for this system. For potassium and cesium microsolvation, dodecahedral cages do occur but their actual structures are far from ideal and their importance appears to be overrated. We offer simple explanations for structural features and trends, and for magic numbers smaller than 20.