Computer Physics Communications 169 (2005) 331–334 www.elsevier.com/locate/cpc On the performance of molecular polarization methods close to a point charge Marco Masia a,* , Michael Probst b , Rossend Rey a a Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, Campus Nord B4-B5, Barcelona 08034, Spain b Institute of Ion Physics, Universität Innsbruck, Technikerstrasse 25, Innsbruck A-6020, Austria Available online 9 April 2005 Abstract The induced dipole moment of a water molecule close to a point charge (with the radius of Na + or Be 2+ ) is computed with the three main methods to implement molecular polarization (point dipoles, fluctuating charges and shell model). The results are compared with high level ab initio calculations and studied as a function of the charge-molecule distance for selected molecular orientations. For the single charge case rather good estimations of the ab initio induced dipole are obtained, with no significant divergences appearing at the shortest distances. Nevertheless, these (linear polarization) methods fail as the charge increases, suggesting that nonlinear effects may not be neglected. Regarding the capabilities of each method, the point dipole method is the one that performs better overall, with the shell model achieving acceptable results in most instances. The fluctuating charge method has some noticeable limitations for implementations with a similar number of interaction sites.  2005 Elsevier B.V. All rights reserved. PACS: 31.15.Kr; 34.20.Gj; 31.15.Kr; 31.15.Qg Keywords: Polarization; Fluctuating charges; Point dipoles; Shell model; Water; Ion; Sodium; Beryllium 1. Introduction The inclusion of molecular polarization plays a cru- cial role in the next generation of force fields for com- puter simulations of molecular systems [1] so that sev- eral, rather different, computational approaches have been developed to model electric polarization. In prin- ciple, any of these methods can at least reproduce * Corresponding author. E-mail address: marco.masia@upc.es (M. Masia). the mean polarizability under homogeneous fields, al- though it is not clear if at short separations their per- formance is still satisfactory. This point is particularly critical since they are routinely used in condensed phase. Moreover, while they are interchangeable at long intermolecular distances, they may have different responses to nonhomogeneous fields. Although com- putational convenience has often been a major factor for the selection of a specific polarization model, the performance at short distances may provide a physi- cally based criterion. 0010-4655/$ – see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cpc.2005.03.074