Chapter 6. Theoretical Determination

Abstract

In Chapter 6, the approaches employed for the theoretical determination of thermodynamic parameters related to ionic solvation are reviewed, and illustrative results of these investigations are discussed, mainly in the restricted context of alkali and halide hydration. The relevant features of the basic physical models employed are presented in Chapter 3, and Chapter 6 is principally concerned with practical issues, improvements and corrections concerning these basic models. The most severe shortcomings of continuum-electrostatic calculations are to neglect the microscopic structure of the solvent molecules and the specific details of ion-solvent interactions, and to rely on the ill-defined concept of an ionic radius. Although numerous types of correction terms have been proposed in the past for various neglected effects, including semi-atomistic approaches, none of these arguably improved models can be considered to be at the same time quantitative and predictive. The most severe shortcomings of classical atomistic simulations are caused by finite-size effects, approximate electrostatics and ambiguity of the electric potential calculation. These issues have, up to recently, prevented the obtension of consistent results for single-ion thermodynamic solvation parameters and air-liquid interfacial potentials. Fortunately, the situation has changed in the past few years with the realization that the corresponding errors could be corrected ex post, so as to achieve methodological independence in the simulation results. Still, the outcome of these calculations remains, even after correction, affected by three major sources of error: the mean-field treatment of electronic polarizability (in most calculations), the approximate representation of van der Waals interactions (functional form and combination rules), and the dependence of the results on the choice of ion-solvent van der Waals interaction parameters, which suffer from a similar kind of ambiguity as the ionic radius of continuum-electrostatics calculations. It is nevertheless possible that the results of simulations concerning a large spectrum of ionic properties, including but extending beyond single-ion solvation free energies, provides in the near future a reliable approach for the accurate evaluation of the experimentally-elusive quantities H,svt, H,svt and χsvt. The corresponding extra-thermodynamic assumption is termed in this book the atomistic-consistency assumption. Finally, quantum-mechanical computations, including quasi-chemical theory, hybrid quantum-classical approaches and Car-Parrinello molecular dynamics simulations, have the major shortcoming of being computationally expensive, which results in practice nowadays in severe restrictions concerning the system size, configuration sampling, basis-set size and treatment of electron correlation. As a result, the computation of single-ion solvation free energies with a sufficient accuracy represents a considerable challenge. However, the steady increase in computing power along with recent methodology developments in the area suggest that the calculation of ionic solvation parameters may become feasible with a sufficient accuracy in the coming decades.