Molecular dynamics investigations of glassforming binary Lennard-Jones systems with different anharmonicities Patrice Bordat a, * , Fre ´de ´ric Affouard b , Marc Descamps b , K.L. Ngai c a Laboratoire de Chimie The ´orique et de Physico-Chimie Mole ´culaire, Department of Chemistry, IFR – rue Jules Ferry, UMR 5624, Universite ´ de Pau et des Pays de l’Adour, BP 27540, 64000 Pau, France b Laboratoire de Dynamique et Structure des Mate ´riaux Mole ´culaires, UMR 8024, Universite ´ Lille I, 59655 Villeneuve d’Ascq cedex, France c Naval Research Laboratory, Washington, DC 20375-5320, USA Available online 14 August 2006 Abstract Dynamical properties of three glassforming binary Lennard-Jones systems with different anharmonicities of the interaction potential have been studied by molecular dynamics simulations. The onset temperature of slow dynamics T A , the Mode Coupling theory critical temperature T C and the Vogel–Fulcher–Tamman temperature T VFT have been determined for the three models. A good agreement is found with most of the predictions of the Mode Coupling Theory. However, a detailed analysis of the master curve obtained from the superposition of the intermediate scattering functions reveals a significative temperature dependence of the stretched exponent b over a wide temperature range from the liquid state to the glass transition. It is shown that both fragility and stretched exponent b unambi- gously correlate with anharmonicity of the interaction potential. Ó 2006 Elsevier B.V. All rights reserved. PACS: 61.20.Ja; 64.60.Ht; 64.70.Pf Keywords: Glass transition; Molecular dynamics; Fragility; Structural relaxation 1. Introduction Understanding of the mechanisms leading to the glass transition has been a subject of intense research in the last decades and it is still a matter of numerous theoretical and experimental studies [1]. Much attention has been recently devoted to the microscopic origin of the very large varia- tions of the fragility m observed between very different glassforming systems: molecular liquids, polymers, ionics materials or orientationally disordered crystals. Indeed, the fragility m offers a useful measure of the temperature dependence of the mobility which is the intrinsic parameter of glass formation phenomena. m can be usually calculated from the slope of the viscosity g or a characteristic relaxa- tion time s at the glass transition T g [2–4]. m ¼ d logðgÞ dðT g =T Þ     T g =T ¼1 or m ¼ d logðsÞ dðT g =T Þ     T g =T ¼1 : ð1Þ Where T g is classically defined as the temperature for which s = 10 2 s or g = 10 13 P. It should be pointed out that while T g is unambiguously defined by the value of the relaxation time, the corresponding viscosity depends on the shear modulus through the Maxwell relation, which can be quite different for polymers and molecular liquids, for instance. The fragility m spreads from 17 for strong glass-formers (having nearly Arrhenius scaled temperature dependence of g or s) such as silica up to 200 for fragile ones such as polymers or low weight molecular liquids. m has been already correlated with thermodynamical or dynamical properties [5–7], or even recently with the vibrational prop- erties of the glass well below T g by Scopigno et al. [8]. In a previous study [9], we investigate some properties of binary Lennard-Jones glassforming systems similar to the 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.109 * Corresponding author. Tel.: +33 559407857; fax: +33 559407862. E-mail address: patrice.bordat@univ-pau.fr (P. Bordat). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 4630–4634