Structures and energetics of CO 2 –Ar n clusters (n = 1–21) based on a non-rigid potential model M. Böyükata, E. Borges, J.C. Belchior, and J.P. Braga Abstract: Energetics and possible stable structures of CO 2 –Ar n (n = 1–21) clusters are investigated by performing molecular-dynamics simulations. The pairwise-additive approximation is tested to construct the potential energy func- tion for describing the non-rigid particle interactions in the system. A potential model by Pariseau et al. (Journal of Chemical Physics, Vol. 42, p. 2335, 1965) is used for the internal motion of the CO 2 molecule and the Billing form potential (Chemical Physics, Vol. 185, p. 199, 1994) is used for all other pair interactions. The stable configurations are determined for the ground state of CO 2 –Ar n clusters, and the growing pattern process of the clusters is determined via rearrangement collisions. Ar atoms tend to surround the CO 2 molecule, and the clusters prefer to form three- dimensional compact structures. Obtained structures and energetics are in quantitative agreement with previous results (Journal of Chemical Physics, Vol. 109, p. 1343, 1998) that have used split–repulsion and ab initio potentials in which the molecule was treated as rigid. Key words: argon, CO 2 , cluster, potential energy function, molecular dynamics. Résumé : Faisant appel à des simulations de dynamique moléculaire, on a étudié les énergies et les structures stables possibles d’agrégats de CO 2 –Ar n (n = 1–21). On a évalué une approximation additive par paire pour construire la fonc- tion d’énergie potentielle qui permettrait de décrire les interactions de particules non-rigides dans le système. On a uti- lisé un modèle de potentiel proposé par Pariseau et al. (Journal of Chemical Physics, numéro 42, p. 2335, 1965) pour le mouvement interne des molécules de CO 2 et la forme de potentiel de Billing (Chemical Physics, numéro 185, p. 199, 1994) pour toutes les autres paires d’interactions. On a déterminé les configurations stables pour l’état fonda- mental des agrégats de CO 2 –Ar n et le processus du patron de croissance des agrégats a été déterminé par le biais de collisions de réarrangement. Les atomes d’argon tendent à entourer les molécules de CO 2 et les agrégats forment de préférence des structures compactes tridimensionnelles. Les structures qui ont été calculées et leurs niveaux d’énergie sont en accord quantitatif avec les résultats rapportés antérieurement (Journal of Chemical Physics, numéro 109, p. 1343, 1998) sur la base d’une répulsion divisée et de potentiels ab initio dans lesquels la molécule était traitée comme rigide. Mots-clés : argon, CO 2 , agrégat, fonction d’énergie potentielle, dynamique moléculaire. [Traduit par la Rédaction] Böyükata et al. 55 Introduction There has been considerable interest in the study of atomic and molecular clusters because of their potential ap- plication in the development of new type of materials (nanoparticles) (1–3). However, one of the main problems in the field of cluster science is describing the geometries of in- termediate systems. In general, it is difficult to find any global minimum structure for molecular clusters. Even for simpler pair potentials, the number of local minima on the potential energy hyper-surface grows exponentially with in- creasing cluster size. Efficient methods have been used to study complex sys- tems. For example, genetic algorithms (GA) have been ap- plied to calculate the lowest energy for a specified potential energy function (PEF) (4, 5). Accurate results compared with ab initio data have shown that the approach can be used with some confidence (5, 6). Other methods such as basin hopping (BH) (7–9) have been widely used for determining global minima, such as using three empirical potentials for describing Si n clusters (10). Goedecker has also suggested minima hopping (MH) as an alternative search method for complex molecular systems (11). Similarly, simulated an- nealing (SA) has also been employed to investigate closed- shell systems (12). Additionally, other strategies can be pro- posed as complementary tools for determining global min- ima of hypersurfaces as efficiently as possible. Particularly for pure atomic and alloy systems, there are useful cluster studies, involving rare-gas atoms bound to a chromophore (e.g., HF and HCl) (13–16) for investigating solvation phenomena at the microscopic level (17, 18). The interactions of CO (17) and CO 2 (18) with rare-gas atoms have also received considerable attention because of their at- mospheric importance (18). For example, extensive experi- mental studies of the CO 2 –Ar system (19–25) and IR spectra Can. J. Chem. 85: 47–55 (2007) doi:10.1139/V06-178 © 2007 NRC Canada 47 Received 12 September 2006. Accepted 3 November 2006. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 9 February 2007. M. Böyükata, 1 E. Borges, J.C. Belchior, 2 and J.P. Braga. Departamento de Química – ICEx, Universidade Federal de Minas Gerais, Pampulha, (31.270–901) Belo Horizonte, MG, Brazil. 1 Present address: Department of Physics, Bozok University, 66100, Yozgat, Turkey. 2 Corresponding author (e-mail: jadson@ufmg.br).