In the framework of the binary quantum cluster equilibrium theory, we introduce a cluster approach to access activity coefficients of binary mixtures. This approach allows derivation of activity coefficients based on quantum chemically calculated clusters. The cluster sets in this work comprise clusters ranging in size from one to six molecules of either a single or two species. For each cluster size up to five conformers if detectable are considered such that important liquid motifs are included. Via self-consistentfield calculations the binary quantum cluster equilibrium theory gives the Gibbs energies and thus the excess Gibbs energies of mixing. Derivation with respect to the particle number allows access to activity coefficients. To achieve an analytical expression, we apply the standard approach of fitting a Redlich−Kister polynomial to the excess Gibbs energy and calculate its derivative, which leads to good results for the binary mixture cases of acetonitrile/benzene and methanol/ethanol, as well as satisfying results for acetone/chloroform. Very good results for vaporization enthalpies are obtained for the pure substances.

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The hydrogen bond network of different small alcohols is investigated via cluster analysis. Methanol/alcohol mixtures are studied with increasing chain length and branching of the molecule. Those changes can play an important role in different fields, including solvent and metal extraction. The extended tight binding method GFN2‐xTB allows the evaluation and geometry optimization of thousands of clusters built via a genetic algorithm. Interaction energies and geometries are evaluated and discussed for the neat systems. Thermodynamic properties, such as vaporization enthalpies and activity coefficients, are calculated with the binary quantum cluster equilibrium (bQCE) approach using our in‐house code peacemaker 2.8. Combined distribution functions of the distances against the angles of the hydrogen bonds are evaluated for neat and mixed clusters and weighted by the equilibrium populations achieved from bQCE calculations.

Binary mixtures of hexafluoroisopropanol with either methanol or acetone are analyzed via classical molecular dynamics simulations and quantum cluster equilibrium calculations. In particular, their populations and thermodynamic properties are investigated with the binary quantum cluster equilibrium method, using our in-house code Peacemaker 2.8, upgraded with temperature-dependent parameters. A novel approach, where the final density from classical molecular dynamics, has been used to generate the necessary reference isobars. The hydrogen bond network in both type of mixtures at molar fraction of hexafluoroisopropanol of 0.2, 0.5, and 0.8 respectively is investigated via the molecular dynamics trajectories and the cluster results. In particular, the populations show that mixed clusters are preferred in both systems even at 0.2 molar fractions of hexafluoroisopropanol. Enthalpies and entropies of vaporization are calculated for the neat and mixed systems and found to be in good agreement with experimental values.