Cluster collisions of water tetramers: a classical dynamical study A. Vegiri a , S.C. Farantos b, * a National Hellenic Research Foundation, Institute of Theoretical and Physical Chemistry, 48 Vas. Constantinou Avenue, Athens 11635, Greece b Department of Chemistry, University of Crete, and Institute of Electronic Structure and Laser, Foundation for Research and Technology ± Hellas, Iraklion, Crete 711 10, Greece Received 16 May 2000; in ®nal form 20 September 2000 Abstract The collision of two water tetramers is investigated with classical molecular dynamics and an empirical potential function. Varying the impact parameter, collision and internal energies of the two colliding tetramers we explore the possibility of forming the cubic structure isomers of the octamer. We ®nd that such events are favored under small collision energies and impact parameters with cold, low internal energy, tetramers. Large impact parameter collisions may lead to an orbiting pair of the two tetramers. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Water clusters remain an important topic of research, both experimentally and theoretically, in spite of the extensive work carried out in the last decades [1]. High quality ab initio calculations for the identi®cation of the global and local minima as well as transition states of small water aggregates have been performed by several groups [2±5]. At the same time sophisticated experiments [6±10] made possible the identi®cation of the minimum energy structures predicted by the ab initio meth- ods. Indeed, trimers, tetramers, pentamers, hexa- mers [6±8], heptamers [10] and octamers [4,9] of water molecules have been isolated and their structures have been characterized in detail. From the knowledge accumulated up to now, there is no doubt that the lowest energy minima of small water clusters with a number of monomers n 6 5 are cyclic, quasi-planar rings. The transition to three-dimensional structures occurs for the hex- amer for which the cage-like geometry is consid- ered as the global minimum of the complex and this is supported by both experiment and quantum Monte Carlo simulations [11]. Analytical functions which describe the water± water interactions in a cluster have also been used in investigating the most stable structures of these systems. In spite of the fact that these functions have been constructed mainly by ®tting data from the bulk phases of water, some of the predicted global minima of small water complexes are cor- rect. Such an analytical function has been used by us in previous studies [12±16]. The potential Chemical Physics 262 (2000) 337±347 www.elsevier.nl/locate/chemphys * Corresponding author. Tel.: +30-81-391-813; fax: +30-81- 391-305. E-mail address: farantos@iesl.forth.gr (S.C. Farantos). 0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII:S0301-0104(00)00316-5