The reaction enthalpy of hydrogen dissociation calculated with the Small System Method from simulation of molecular fluctuations

{We show how we can find the enthalpy of a chemical reaction under non-ideal conditions using the Small System Method to sample molecular dynamics simulation data for fluctuating variables. This method, created with Hill's thermodynamic analysis, is used to find properties in the thermodynamic limit, such as thermodynamic correction factors, partial enthalpies, volumes, heat capacities and compressibility. The values in the thermodynamic limit at (T,V, mu(j)) are then easily transformed into other ensembles, (T,V,N-j) and (T,P,N-j), where the last ensemble gives the partial molar properties which are of interest to chemists. The dissociation of hydrogen from molecules to atoms was used as a convenient model system. Molecular dynamics simulations were performed with three densities; rho = 0.0052 g cm(-3) (gas)