Nature of Halide-Water Interactions: Insights from Many-Body Representations and Density Functional Theory

Interaction energies of halide-water dimers, X^-(H₂O), and trimers, X^-(H₂O)₂, with X = F, Cl, Br, and I, are investigated using various many-body models and exchange correlation functionals selected across the hierarchy of density functional theory (DFT) approximations. Analysis of the results obtained with the many-body models demonstrates the need to capture important close-range interactions in the regime of large intermolecular orbital overlap, such as charge transfer and charge penetration. Failure to reproduce these effects can lead to large deviations relative to reference data calculated at the coupled cluster level of theory. Decompositions of interaction energies carried out with the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA) method demonstrate that permanent and inductive electrostatic energies are accurately reproduced by all classes of XC functionals (from generalized gradient corrected (GGA) to hybrid and range-separated hybrid functionals), while significant variance is found for charge transfer energies predicted by different XC functionals. Since GGA and hybrid XC functionals predict the most and least attractive charge transfer energies, respectively, the large variance is likely due to the delocalization error. In this scenario, the hybrid XC functionals are then expected to provide the most accurate charge transfer energies. The sum of Pauli repulsion and dispersion energies is the most varied among the XC functionals, but it is found that a correspondence between the interaction energy and the ALMO-EDA total frozen energy may be used to determine accurate estimates for these contributions.