ACADEMIA ROMÂNĂ Revue Roumaine de Chimie http://web.icf.ro/rrch/ Rev. Roum. Chim., 2012, 57(4-5), 463-468 Dedicated to Professor Ionel Haiduc on the occasion of his 75 th anniversary THEORY AND SIMULATION OF TRANSPORT PROPERTIES FOR PENETRABLE-SPHERE MODEL SYSTEMS: I. SELF-DIFFUSION Viorel CHIHAIA, a Gabriel MUNTEANU, a Vlad T. POPA a and Soong-Hyuck SUH b,* a “Ilie Murgulescu” Institute of Physical Chemistry, Roumanian Academy, 202 Splaiul Independentei, Bucharest 060021, Roumania b Department of Chemical Engineering, Keimyung University, Daegu 704-701, Korea Received February 1, 2012 Molecular dynamics simulations have been performed to investigate the transport properties of self-diffusion coefficients in the penetrable-sphere model fluid. With increasing system densities, simulation results for the product of the packing fraction and the self-diffusion coefficient exhibit the transitional behavior: from a nearly independent function of density in lower repulsive systems, where the soft-type collisions are dominant, to a rapidly decreasing function in higher repulsive systems, where most particle collisions are the hard-type reflections due to the low-penetrability effects. In the systems of highly repulsive energy barriers, a poor agreement with theoretical and empirical predictions is observed even at moderate densities due to the cluster-forming structure and the phase transition from the fluid-like to the solid-like state. INTRODUCTION * When Professor Ionel Haiduc visited Korea in 2009 for Korea/Romania Joint Workshop on Molecular Science and Engineering, gave a special lecture on the subject of molecular and supramolecular chemistry. 1 A current progress in this research area, although not directly addressed in his lecture, has been associated with computer-based molecular simulations. In this way, new supramolecular materials, based on computer modeling and simulation techniques, can be fabricated with specific molecular structures in the processes of molecular recognition and supramolecu- lar organization. Other simulation topics were also discussed with Professor Haiduc for further science and engineering applications demonstrated in the KISTI Supercomputing Center, Korea. Apart from some initial system parameters, only an input requirement is the model interaction * Corresponding author: shsuh@kmu.ac.kr potential in order to perform the standard classical computational method including Monte Carlo, Brownian dynamics, and molecular dynamics calculations. 2 For hard condensed matter, the steeply repulsive short-ranged potential rarely allows the overlapping configuration between two colliding particles, e.g., the hard-sphere and the Lennard-Jones potential. However, as observed even at the room temperature in the systems of molecular aggregates such as micelles and polymer chains in solution, the center of mass of two constitutive molecules can penetrate each other due to the weak interactions among structural elements and the delicate balance between the entropic and the enthalpic contributions to the free energy. In such soft condensed matter systems, 3 it is often represented by the ultrasoft bounded potential, i.e., the effective interaction parameter. For theoretical and simulation investigations in soft condensed matter, one of the simplest and com-