Density fluctuations in the liquid Na–Cs alloy. An ab initio molecular dynamics study J. Blanco a , D.J. Gonz alez a, * , L.E. Gonz alez a , J.M. L opez a , M.J. Stott b a Departamento de F  ısica Te orica, Facultad de Ciencias, Universidad de Valladolid, 47011 Valladolid, Spain b Department of Physics, Queen’s University, Kingston, Ont., Canada K7L 3N6 Abstract WepresentresultsforseveralstructuralanddynamicalpropertiesofliquidNa–Csalloys.Thestudyhasbeencarried out using orbital free ab initio molecular dynamics, combined with local ionic pseudopotentials constructed within the same framework. The results show good agreement with the available experimental data, reproducing the phase sep- arating tendency exhibited by this alloy. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.25.Mv; 61.55.Hg; 61.20.Ja 1. Introduction Molecular dynamics (MD) has become a useful technique for studying the properties of liquid systems. Whereas classical molecular dynamics (CMD) techniques require interatomic potentials, theabinitiomoleculardynamics(AIMD)methods compute the forces acting on the nuclei from electronic structure calculations performed as the MD trajectory is generated; the nuclear positions evolve according to classical mechanics, while the electronic subsystem follows adiabatically. The Na–Cs liquid alloy system exhibits strong homo-coordinating tendencies and has attracted much experimental [1,2] and theoretical [2–5] work. Among the experimental work we mention the measurement of several thermodynamic quan- tities [1], as well as the static structure factor for several concentrations by X-ray and neutron dif- fraction experiments [2]. A marked departure from ideality has been observed in several properties; in particular, there is a large increase in the con- centration–concentration structure factor, S cc ð0Þ, at sodium concentrations, x Na  0:8. On the theo- retical side, several studies using either semiem- pirical [3,4] or more fundamental [2,5] approaches have attempted to calculate the static structure and/or some thermodynamic quantities. Semiem- piricalapproachesusuallyresortedtoeitherahard- sphere description [3] or some thermodynamic model [4] of the alloy providing useful insight into the interatomic interactions in the alloy. More fundamental approaches include the studybyHuijbenetal.[2],whoperformedaCMD simulation using interatomic potentials obtained Journal of Non-Crystalline Solids 312–314 (2002) 148–152 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +34-983 423 147; fax: +34-983 423 013. E-mail address: david@liq1.fam.cie.uva.es (D.J. Gonza ´lez). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(02)01670-8