Larger water clusters with edges and corners on their way to ice: Structural trends elucidated with an improved parallel evolutionary algorithm

For the difficult task of finding global minimum energy structures for molecular clusters of nontrivial size, we present a highly efficient parallel implementation of an evolutionary algorithm. By completely abandoning the traditional concept of generations and by replacing it with a less rigid pool concept, we have managed to eliminate serial bottlenecks completely and can operate the algorithm efficiently on an arbitrary number of parallel processes. Nevertheless, our new algorithm still realizes all of the main features of our old, successful implementation. First tests of the new algorithm are shown for the highly demanding problem of water clusters modeled by a potential with flexible, polarizable monomers (TTM2-F). For this problem, our new algorithm not only reproduces all of the global minima proposed previously in considerably less CPU time but also leads to improved proposals in several cases. These, in turn, qualitatively change our earlier predictions concerning the transitions from all-surface structures to cages with a single interior molecule, and from one to two interior molecules. Furthermore, we compare preliminary results up to n = 105 with locally optimized cuts from several ice modifications. This comparison indicates that relaxed ice structures may start to be competitive already at cluster sizes above n = 90.