i-TTM Model for Ab Initio-Based Ion–Water Interaction Potentials. 1. Halide–Water Potential Energy Functions

Abstract

New potential energy functions (i-TTM) describing the interactions between halide ions and water molecules are reported. The i-TTM potentials are derived from fits to electronic structure data and include an explicit treatment of two-body repulsion, electrostatics, and dispersion energy. Many-body effects are represented through classical polarization within an extended Thole-type model. By construction, the i-TTM potentials are compatible with the flexible and fully ab initio MB-pol potential, which has recently been shown to accurately predict the properties of water from the gas to the condensed phase. The accuracy of the i-TTM potentials is assessed through extensive comparisons with CCSD(T)-F12, DF-MP2, and DFT data as well as with results obtained with common polarizable force fields for X(-)(H2O)n clusters with X(-) = F(-), Cl(-), Br(-), and I(-), and n = 1-8. By construction, the new i-TTM potentials will enable direct simulations of vibrational spectra of halide-water systems from clusters to bulk and interfaces.

Absolute proton hydration free energy, surface potential of water, and redox potential of the hydrogen electrode from first principles: QM/MM MD free-energy simulations of sodium and potassium hydration

“…In this respect, it is interesting to note that three recent approaches for improving the description of ion-water interactions at the MM level rely in effect on a weakening of the ion-solvent Coulombic interactions. These are: the induction-type 66,67,104,107,[278][279][280][281]284 force fields, where CT effects are included explicitly, the molecular dynamics in electric continuum (MDEC) approach, [285][286][287] where the ion charge is effectively scaled 75,[77][78][79]288 by a factor of about 0.7-0.9 (in line with QM results 282 ), and the multisite ion description, [289][290][291][292][293][294][295] where the ion charge is redistributed over virtual off-center sites within the ion. The observation made here that the features of the QM/MM RDFs can be reproduced more accurately with a MM model when reducing the nominal charge of the ion to +0.75 e is perfectly in line with these considerations.…”

“…16,17,[60][61][62][63][64][65][66][67] In this case, the solvent is represented in its atomistic granularity and special care must be taken to remain compatible with the Born ETA. Although the issue has given rise to much debate (see, e.g., the Appendix of Ref.…”

“…Just like ionic radii, these are elusive quantities, as can be judged by the numerous parametrizations available. 1,57,59,63,[65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] The latter issue could in principle be addressed by moving to quantum-mechanical (QM) models, where the ionsolvent interactions are determined on the basis of fundamental physics instead of empirical parameters. In addition, the QM description is expected to further improve the representation of short-range ion-solvent interactions (more accurate description of specific ion-solvent interactions, from pairwiseadditive in MM to now include electronic polarization, charge transfer, and other many-body effects).…”

“…For the cluster-continuum calculations, error sources include the selection of the clusters (sampling), their small sizes (smooth trends and fast convergence being required for a meaningful extrapolation 117 ), the estimation of the cluster entropy, 118-120 the choice of a cluster permittivity in the CE calculation, and the problems associated with a QM-CE boundary (surface definition, atomic radii). For the CPMD/BOMD calculations, error sources include the shortcomings of DFT methods in terms of electron correlation (including dispersion), affecting both bulk water properties [121][122][123][124][125][126][127][128][129][130] and ion-water interactions, 66,67,105,131 the small system sizes considered (resulting in a high weight for the approximate finite-size correction term), the very limited sampling times, the ambiguity in defining the appropriate reference potential within DFT water, 21,112,[132][133][134][135][136] and the numerical problems (see, e.g., footnotes 42 and 45 in Ref. 111) associated with creating a species or injecting electrons when applying computational alchemy in a QM framework [137][138][139][140][141][142][143] (see, however, Refs.…”

“…Although the MP2 level of theory is only the first scheme in a hierarchy of approaches to account for electron-correlation effects, 179 it has been shown to provide a good compromise between accuracy and computational effort in the context of ion-water interactions. 66,67,77,78,130,155,180,181 Recent simulations of the bulk water dynamics at this level of theory [123][124][125][126][127] also suggest that it is adequate for the representation of the liquid properties.…”