Molecular Simulation of Chemical Reaction Equilibriumby Computationally Efficient Free Energy Minimization

摘要

The molecular simulation of chemical reaction equilibrium (CRE) is a challenging and important problem of broad applicability in chemistry and chemical engineering. The primary molecular-based approach for solving this problem has been the reaction ensemble Monte Carlo (REMC) algorithm [Turner et al. Molec. Simulation 2008, 34, (2), 119−146 []], based on classical force-field methodology. In spite of the vast improvements in computer hardware and software since its original development almost 25 years ago, its more widespread application is impeded by its computational inefficiency. A fundamental problem is that its MC basis inhibits the implementation of significant parallelization, and its successful implementation often requires system-specific tailoring and the incorporation of special MC approaches such as replica exchange, expanded ensemble, umbrella sampling, configurational bias, and continuous fractional component methodologies. We describe herein a novel CRE algorithm (reaction ensemble molecular dynamics, ReMD) that exploits modern computer hardware and software capabilities, and which can be straightforwardly implemented for systems of arbitrary size andcomplexity by exploiting the parallel computing methodology incorporatedwithin many MD software packages (herein, we use GROMACS for illustrativepurposes). The ReMD algorithm utilizes these features in the contextof a macroscopically inspired and generally applicable free energyminimization approach based on the iterative approximation of thesystem Gibbs free energy function by a mathematically simple convexideal solution model using the composition at each iteration as areference state. Finally, we additionally describe a simple and computationallyefficient a posteriori method to estimate the equilibriumconcentrations of species present in very small amounts relative toothers in the primary calculation. To demonstrate the algorithm, weshow its application to two classic example systems considered previouslyin the literature: the N2–O2–NOsystem and the ammonia synthesis system.