Polarization damping in halide–water dimers Marco Masia a, * , Michael Probst b , Rossend Rey a a Departament de Fı ´sica i Enginyeria Nuclear, Universitat Polite `cnica de Catalunya, Campus Nord, Modul B4-B5, 2n pis, Barcelona 08034, Spain b Institute of Ion Physics, Universita ¨ t Innsbruck, Technikerstrasse 25, Innsbruck A-6020, Austria Received 13 October 2005 Available online 20 January 2006 Abstract High level ab initio calculations show that the self-induced dipole moment of a halide–water dimer deviates from the usually employed point dipole model, with a substantial nonlinear damping at separations corresponding to the first hydration shell. The total dipole moment is rather similar along the halide series, with the maximum value decreasing as anionic polarizability increases. A new imple- mentation of the Thole damping method satisfactorily reproduces the dipole moment at all separations for the most probable configurations. Ó 2006 Elsevier B.V. All rights reserved. Halide hydration, an old topic in chemical physics, has regained experimental and theoretical interest. Recently, using ultrafast spectroscopy, it has been possible to directly probe the dynamics of the solvation shell in the bulk [1,2], offering a potential tool for the study of shell exchange [3]. It is the behaviour at the air–water interface, though, that has been the focus of more intense research within the last decade. The initial theoretical [4] and experimental [5] hints of an enhanced anion concentration at the surface, have been recently confirmed experimentally both for halides [6] and for a molecular anion [7]. This finding contradicts the accepted picture for over a century (in which anions would prefer interior solvation) and has far reaching impli- cations for atmospheric chemistry [8,9]. Surface solvation seems to be particularly sensitive to the different types of interactions. Polarization forces are considered the direct cause for the emergence of surface states [10,11], and indeed polarization has also been sug- gested to be relevant for bulk hydration [12]. This crucial role of polarization was already highlighted in the initial work by Perera and Berkowitz [4], and confirmed by a number of simulations (see the review by Jungwirth and Tobias [10] and references within), although it has also been questioned from different angles [13–16]. The evidence for the role of polarization forces stems from classical molecular dynamics (MD) simulations, because in ab initio MD (which also predicts the propensity for the interface [10]) decomposition of the various contri- butions in order to judge the importance of each type of force is not completely straightforward. It is interesting to note that in almost all classical simulations to date polarization has been treated in the same way: the point dipole model of polarization [17] is applied without further refinements (although different implementations of polari- zation are possible, as in Ref. [18], they all share the same basic characteristics [19,27]). In this model, point dipoles are located on the ion and on one or several sites within the molecule. As it is known that classical simulations can be sensitive to the force field parametrization [6], a con- siderable effort is currently directed towards the accurate ab initio calculation of polarizability (the single adjustable parameter) in the liquid phase, starting with that of the water molecule [21,22] and continued with those for halides in water [23,24]. Remarkably, the point dipole model itself has not been questioned, although its limitations are known for the fit- ting of gas phase molecular polarizabilities [17,25] and for the simulation of crystal and molten salts [26]. In both cases 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.12.080 * Corresponding author. Fax: +34 934017100. E-mail addresses: marco.masia@upc.edu (M. Masia), michael.probst @uibk.ac.at (M. Probst), rosendo.rey@upc.edu (R. Rey). www.elsevier.com/locate/cplett Chemical Physics Letters 420 (2006) 267–270