Molecular Dynamics Study of a KCl Aqueous Solution: Dynamical Results M. Migliore Istituto Applicazioni Interdisciplinari Fisica, CNR, 1-90123 Palermo, Italy S. L. Fornili Istitut di Fisica, Universida di Palermo and IAIF-CNR, 1-90123 Palermo, Italy E. Spohr and K. Heinzinger Max-Planck-Institut für Chemie (Otto-Hahn-Institut), D-6500 Mainz, FRG Z. Naturforsch. 42 a, 227-230 (1987); received December 12, 1986 In this paper we report on dynamical properties of a 2.2 molal aqueous KCl solution as obtained from an 8.7 ps MD simulation at an average temperature of 289 K. Velocity autocorrelation functions, self-diffusion coefficients and spectral densities of the hindered translational and librational motions of the ions and the water molecules assigned to three subsystems - hydration water of the cations, hydration water of the anions and bulk water — are discussed. Introduction The interplay of the stereodynamical behaviour of solvent and solute may remarkably affect the functional stability of complex solutes, such as large biomolecules [1, 2]. The detailed study of the inter- action between solvent water and small, yet impor- tant solutes like monovalent ions, which usually are present in biological environments, may help to understand structural and dynamical characteristics of biomolecular solutions. In the present paper we report results of an analysis of the dynamical prop- erties of an aqueous KCl solution studied by a molecular dynamics (MD) simulation. In a previous paper [3] the structural characteristics of the same solution, averaged over the First 5 ps, have been reported and compared with experimental data. MD Simulation The dynamical properties of a 2.2 molal KCl aqueous solution reported in this paper have been evaluated from an MD simulation which extended over 40000 time steps equivalent to a total elapsed time of 8.7 ps at an average temperature of 289 K. Reprint requests to Dr. M. Migliore, CNR-IAIF, Via Archirafi 36,1-90123 Palermo, Italy. The basic periodic cube contained 200 water mole- cules and eight ions of each kind. The ST 2 water model was used and the ions were modeled as Lennard-Jones (LJ) spheres with an elementary charge in the center. The values of the LJ param- eters and further details concerning the simulation procedure are given in [3]. Velocity Autocorrelation Functions To obtain useful information on the microscopic dynamics of the water molecules and of the ions in solution we describe the particle dynamics by means of the time dependent autocorrelation functions (ACF) NT N C A (0 = (\/NN,) Z Z Aj( ti )  Aj(tj + /), (1) ' j where N denotes the number of particles, N, the number of time averages and A the considered quantity. These functions have been calculated for the ions and separately for the three water sub- systems in the solution (i.e. bulk water and the water molecules belonging to the first hydration shell of the two ionic species). At each reference time each water molecule was assigned to a particular subsystem and this assignment was kept for the entire correlation time. As the residence time 0340-4811 / 87 / 0200-237 $ 01.30/0. - Please order a reprint rather than making your own copy.