Canonical parallel tempering simulations of Ar n –HCl clusters (n ¼ 1–12) Michael F. Russo Jr. 1 , E. Curotto * Department of Chemistry and Physics, Arcadia University, 450 S. Easton Road, Glenside, PA 19038, USA Received 10 July 2003; in final form 22 August 2003 Published online: 16 September 2003 Abstract A systematic investigation of the thermodynamic and structural properties of Ar n –HCl clusters in the n ¼ 1–12 range by classical parallel tempering is carried out. We find considerable solid to solid mixing isomerizations that take place at temperatures well below the melting range. Unlike the related system of Ar n –HF clusters we have studied recently, the HCl is preferentially on the surface at low temperatures (T < 5 K). However, it is thermodynamically feasible for the noble gas framework of sufficient size to cage the relatively large HCl molecule in both a highly disordered solid state and the melting anomaly region. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction In a recent publication [1] we presented the re- sults of a systematic investigation of the thermo- dynamic and spectroscopic properties of small Ar n –HF clusters in the n ¼ 1–12 range. Using structural characterization methods, we learn how these properties are affected by isomerizations. In particular, we find a considerable amount of solid to solid isomerizations taking place at tempera- tures below 1 K for less stable systems. The findings in Ref. [1] suggest that achieving thermal equilib- rium in experiments [2] may be possible for Ar n – HF systems. Therefore, the opportunity exists to compare high-level theory and high-resolution experiments with the common goal of refining potential energy surfaces beyond the three-body interactions for noble gas–hydrogen halide clusters in the near future. The long-term goal of our ap- proach for the theoretical treatment of noble gas– hydrogen halide clusters is to employ path integral simulations [3–7]. For Ar n –HF, it has been rela- tively simple to identify sizes even in experiments with neutral species in helium nanodroplets [2] thanks to the distinct dependence of the red shift on the size of the argon cage. In the Ar n –HCl case there is a need to carry out careful simulations before spectra can be interpreted unambiguously, since the red shit of Ar n –HCl is not as strongly dependent on size. Of course one cannot exclude Chemical Physics Letters 379 (2003) 386–392 www.elsevier.com/locate/cplett * Corresponding author. Fax: +1-215-5727595. E-mail address: curotto@arcadia.edu (E. Curotto). 1 Present address: Department of Chemistry, Pennsylvania State University. 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.08.070