Surface Science Letters Structures of adsorbed water layers on MgO: an ab initio study R.M. Lynden-Bell a, * , L. Delle Site b , A. Alavi c a Atomistic Simulation Group, School of Mathematics and Physics, Queen's University, Belfast BT7 1NN, UK b Max Planck Institute for Polymer Research, P.O. Box 3148, D-55021 Mainz, Germany c University Chemical Laboratory, Lens®eld Road, Cambridge CB2 1EW, UK Received 15 September 2001; accepted for publication 10 October 2001 Abstract A systematic search using an ab initio density-functional method has been carried out for energy minima for a monolayer of water on MgO. Minima were sought in which one third of the water molecules were dissociated, and the observed p3  2) symmetry satis®ed. Six such minima were found, three of which are within 300k B per water molecule of the lowest energy structure. We also found a structure with a similar energy with 2  2) symmetry and half the water molecules dissociated. The structures are stabilised by the donation of three hydrogen bonds to each hydroxide ion. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Chemisorption; Surface structure, morphology, roughness, and topography; Magnesium oxides; Water; Single crystal surfaces 1. Introduction The structure of adsorbed ®lms of water on a perfect surface of MgO has been a matter of con- siderable experimental and computational interest for some years. Experimentally, water is found to adsorb reversibly on a ¯at MgO surface to form a monolayer at temperatures in the region of 200 K. LEED and helium scattering experiments [1±3] indicate a p3  2) structure with glide planes per- pendicular to the long axis of the cell. The revers- ibility of the experiments was interpreted as showing that the monolayer was physisorbed rather than dissociatively chemisorbed. This con- clusion was generally accepted until ab initio calculations indicated spontaneous formation of a structure with one third of the water molecules dissociated [4,5]. This result has been con®rmed by later work [6,7]. Earlier calculations had been performed using classical interaction potentials [10±12] which gave a planar structure for the physisorbed layer with the water oxygens above magnesium ions. The molecules in the monolayer were connected by a two dimensional network of hydrogen bonds and there were no hydrogen bonds to the surface oxide ions. Early ab initio work [9] showed that a single adsorbed water molecule was stable when physi- cally adsorbed was only able to dissociate in the Surface Science 496 2002) L1±L6 www.elsevier.com/locate/susc * Corresponding author. Tel.: +44-2890-335329; fax: +44- 2890-241958. E-mail address: r.lynden-bell@qub.ac.uk R.M. Lynden- Bell). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)01669-7