Accurate Prediction of the Hydration Free Energies of 20 Salts through Adaptive Force Matching and the Proper Comparison with Experimental References

Abstract

Simple pairwise potentials for five alkali ions and four halide ions were developed by only fitting to ab initio MP2 forces with the adaptive force matching (AFM) method. Without fitting to any experimental information, the AFM models predict the hydration free energies of all 10 fluoride and chloride salts formed by these ions within 1.5% of experimental references. The predicted hydration free energies for the 10 bromide and iodide salts are within 5–6% of experimental references with the larger error likely due to the neglect of explicit treatment of polarization and charge transfer. An inconsistency in the treatment of the gas phase entropy term between experimental and theoretical approaches is discussed. A new simplified hydration free energy for the ions is reported for use as a more appropriate experimental reference for further theoretical studies. The simulations show different dipole alignments for the hydration waters of cations and anions. While hydration waters of small cations tend to align their molecular dipole toward the ion, the dipole of one of the water OH bonds is aligned with the field of an anion.

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.…”

“…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 completeness, we note that most MM models rely at least in part on the fitting of ion-solvent parameters against QM calculations considering ion-water dimers or small ion-water clusters/boxes, a fitting that becomes systematic when a force-matching approach is applied. [77][78][79] However, the subsequent MM calculations do not really qualify for being called calculations at the QM level.…”

Development of Nonbonded Models for Metal Cations Using the Electronic Continuum Correction

“…These outcomes can be compared with those of scaled charges nonbonded models taken from the literature. In particular, Li and Wang recently parametrized the same monovalent ions using a pairwise potential based on the Buckingham term and a exponential repulsion term and with an explicit inclusion of polarization effects by using a 0.766 e ionic charge . The predicted IOD values extrapolated from the corresponding rdf profiles are 1.96 å, 2.35 å, and 2.77 å, for Li + , Na + , and K + , respectively.…”

“…In particular, Li and Wang recently parametrized the same monovalent ions using a pairwise potential based on the Buckingham term and a exponential repulsion term and with an explicit inclusion of polarization effects by using a 0.766 e ionic charge. [25] The predicted IOD values extrapolated from the corresponding rdf profiles are 1.96 Å, 2.35 Å, and 2.77 Å, for Li + , Na + , and K + , respectively. Thus, the ECC model here optimized offer good performances in the reproduction of geometric properties even using a simpler functional form respect to the one cited previously.…”