(183e) Linking the COSMO-RS Model to Molecular Dynamics Simulations

We describe how to link the results of the Conductor-like Screening Model for Real Solvents (COSMO-RS) model, a powerful tool for calculating thermodynamics equilibrium properties of organic compounds, with Molecular Dynamics (MD) simulations. In the COMSO-RS model, a liquid is regarded as a corresponding ensemble of pairwise-interacting surface segments whose charges are obtained from quantum mechanical COSMO calculations. A self-consistent solution of the partition function for calculating the chemical potential is enabled by applying the free segment approximation, in which a molecule’s surface is divided into segments, which can be paired independently. Despite the loss of steric information of the system, COSMO-RS provides reliable results for small molecules, but is less reliable for more complex molecules.

To better understand the free segment approximation and interacting surface segments, various compounds widely used in chemical engineering, such as water, hexane, diethyl ether, acetone, toluene, and ethanol, were studied in MD simulations using the LAMMPS Molecular Dynamics Simulator and the OPLS-AA force field. By examining the liquid structure using weighted Voronoi tessellation, a convenient definition for surface contact in molecular simulations is established. According to the defined criteria, histograms describing the probability that the surfaces of certain atoms are in contact can be created. Comparing these results to histograms based on COSMO-RS calculations identifies a link between the two vastly different simulation approaches. This link will pave the way for future studies on systems in which COSMO-RS does not provide accurate results and should aid in identifying COMSO-RS’s inadequacies as well as suggest possible improvements.