Molecular dynamics simulations of solid–liquid phase transition in small water aggregates Andrei V. Egorov a, * , Elena N. Brodskaya b , Aatto Laaksonen c a Institute of Physics, St. Petersburg University, 198504 St. Petersburg, Russia b Institute of Chemistry, St. Petersburg University, 198504 St. Petersburg, Russia c Division of Physical Chemistry, Arrhenius Laboratory, Stockholm University, S 106 91 Stockholm, Sweden Received 16 July 2004; received in revised form 9 November 2004; accepted 22 November 2004 Abstract Small water clusters, (H 2 O) 20 and (H 2 O) 26 , containing charged (Li + ) and neutral (atomic Li and methane) particles, have been studied using molecular dynamics simulations at temperatures ranging from 10 to 200 K. Transitions between solid and liquid phases were investigated in relation to inserted particle charge and size, as well as the initial cluster configuration. It was found that embedding the methane molecule inside a dodecahedral (H 2 O) 20 cage significantly stabilizes the cluster configuration. Nevertheless, the melting temperature of the CH 4 (H 2 O) 20 cluster is reduced by methane. This system does not underwent an additional polymor- phic structural transformation typical for the pure water dodecahedron, resulting in lower total energy. In the case of the 3 · 3 · 3 cubic-ice aggregate the enclosure of a neutral molecule also leads to a stabilization of the cluster structure and the melting temper- ature of CH 4 (H 2 O) 26 or Li 0 (H 2 O) 26 aggregates raises. However, the main features of the overall phase evolution are mainly determined by water–water interactions. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 31.15.Qg; 36.40.Àc; 36.40.Qv; 36.40.Wa Keywords: Molecular simulations; Solid–fluid equilibria; Water clusters; Hydrates 1. Introduction Understanding the mechanisms for nucleation, growth, melting, and freezing in small water aggregates are problems of fundamental interest in chemistry, physics and environmental science. Water clusters play an important role in many areas from biochemical processes to atmospheric chemistry and gas (clathrate) hydrates forming. In spite of considerable experimental and theoretical efforts, several problems are still unsolved. Among them, the description of molecular mechanisms for solid–liquid phase transitions in water aggregates and the effect of inserted particles on cluster structural transformations require more detailed con- sideration. Experimental studies of small systems encounter still many difficulties. The quantum chemical calculations of configurational changes in clusters with dozens molecules are computationally very expen- sive. Currently such clusters can be effectively modeled only via molecular dynamics (MD) or Monte-Carlo computer simulations, both capable to provide detailed information of all atoms in the simulated system. However, the computer simulations are based on empir- ical man-made potential models describing the inter- actions between the molecules and the accuracy of 0927-0256/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2004.11.015 * Corresponding author. Tel.: +7 812 428 4362; fax: +7 812 428 7240. E-mail address: egorov@esr.phys.spbu.ru (A.V. Egorov). www.elsevier.com/locate/commatsci Computational Materials Science 36 (2006) 166–170