Validation of the COSMO-RS electrostatics by Monte-Carlo simulations

Abstract

The conductor-like screening model for real solvents (COSMO-RS) meanwhile is widely accepted as a novel and efficient tool for the predictive simulation of fluid phase thermodynamics based on quantum chemical calculations for the individual compounds. In COSMO-RS all molecular interactions are described as local interactions of surface segments and quantified by the local surface polarization charge densities which are known from the quantum chemical simulation of the compounds in a virtual conductor (COSMO). The statistical thermodynamics of the interacting segments is solved in COSMO-RS by an accurate and efficient self-consistency algorithm. While the treatment of hydrogen bonding and dispersive interactions as local surface interactions appears to be justified by the local nature of these interactions, the local description of the electrostatics in COSMO-RS is somewhat counter-intuitive, because the Coulomb electrostatics is longer ranged and non-local in its nature. This has been a frequent topic of criticism of the COSMO-RS theory. In order to check the validity of the local description of electrostatics we compare the results of MC simulations and COSMO-RS calculations for dipolar and quadrupolar compounds of the same shape and very similar σ-profiles. According to the COSMO-RS theory such two compounds should have very similar interactions in the liquid phase, which is a bit counterintuitive from the perspective of the conventional molecular simulation. For this purpose different simplified CO 2-shaped and spherical pseudo-molecules with a variety of different charge distributions have been consistently treated by COSMO-RS and by MC simulation applying a 2-center or 1-center Lennard–Jones model, respectively, in the MC-simulations. The comparison reveals that the results for the average electrostatic interaction energies and for the vapour pressures resulting from both simulation methods are in satisfying agreement. This indicates the validity of the local description of electrostatics and hence gives further confirmation of the fundaments of the COSMO-RS theory.