1 Thermodynamic Properties of Hydrogen Dissociation Reaction from the Small System Method and Reactive Force Field ReaxFF Thuat T. Trinh, 1,2 Nora Meling, 2 Dick Bedeaux 2 and Signe Kjelstrup 2,3* 1 Computing and Design BioNanoChemistry Research Group, Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam 2 Department of Chemistry, NTNU, Norwegian University of Science and Technology, Høgskoleringen 5, 7491 Trondheim, Norway. 3 Department of Microelectronics, Delft University of Technology, Delft, The Netherlands * E-mail: signe.kjelstrup@ntnu.no ABSTRACT We present thermodynamic properties of the H 2 dissociation reaction by means of the Small System Method (SSM) and Reactive force field ReaxFF simulations. Thermodynamic correction factors, partial molar enthalpies and heat capacities of` the reactant and product were obtained in the high temperature range; up to 30000K. The results obtained by ReaxFF potential agree well with previous results obtained with a three body potential (TBP). This indicates that the popular reactive force field method can be well combined with the newly developed SSM for large-scale simulations of chemical reactions. The approach may be useful in the study of heat and mass transport in combination with chemical reactions. Keywords Thermodynamics, Small System Method, Hydrogen Dissociation, Reactive force field, Molecular Dynamics INTRODUCTION Thermodynamic properties of reactions play a central role in chemistry and chemical engineering. These properties are used to describe macroscopic systems, such as in a laboratory and in chemical reactors. These classical descriptions fail at the nanoscale because thermodynamic variables are no longer extensive for a few particles. Schnell et al. [1] have used a new scaling law in the development of the Small System Method (SSM), which connects properties of the system in the nanoscale with macroscopic limit. SSM has been successfully used to calculate thermodynamic correction factors (or simply thermodynamic factors), derivatives of activity coefficients with respect to the composition, partial molar enthalpies, partial molar volumes, and reaction enthalpies of macroscopic systems. The results have been in excellent agreement with data obtained from other methods [2]. The calculation of thermodynamic factors (derivative of activity coefficient with respect to composition) or so-called Kirkwood-Buff integrals [3] are of great importance to quantify diffusion. SSM was also applied to find thermodynamic factors for Li + ions in solid state conductors [4], alkane in carbon nanotubes [5] and particles in nanopores [6]. The thermodynamic factor allows the calculation of Maxwell-Stefan diffusion coefficients from Fick diffusion coefficients, and vice versa, for binary, even ternary systems [7,8]. Most of these computation studies were