Rational design of ion force fields based on thermodynamicsolvation properties

摘要

Most aqueous biological and technological systems contain solvated ions. Atomistic explicit-watersimulations of ionic solutions rely crucially on accurate ionic force fields, which contain mostcommonly two adjustable parameters: the Lennard-Jones diameter and the interaction strength.Assuming these parameters to be properly optimized, the plethora of parameters one finds in theliterature for one and the same ion is surprising. In principle, the two parameters should be uniquelydetermined by matching two ionic properties obtained for a particular water model and within agiven simulation protocol with the corresponding experimental observables. Traditionally, ionparameters were chosen in a somewhat unsystematic way to reproduce the solvation free energy andto give the correct ion size when compared with scattering results. Which experimental observableone chooses to reproduce should in principle depend on the context within which the ionic forcefield is going to be used. In the present work we suggest to use the solvation free energy inconjunction with the solvation entropy to construct thermodynamically sound force fields for thealkali and halide ions for the simulation of ion-specific effects in aqueous environment. To that endwe determine the solvation free energy and entropy of both cations and anions in the entire relevantparameter space. As an independent check on the quality of the resulting force fields we alsodetermine the effective ionic radius from the first peak of the radial ion-water distribution function.Several difficulties during parameter optimization are discussed in detail. (i) Single-ion solvationdepends decisively on water-air surface properties, which experimentally becomes relevant whenintroducing extrathermodynamic assumptions on the hydronium (H30+) solvation energy. Fittingion pairs circumvents this problem but leaves the parameters of one reference ion (here we choosechloride) undetermined. (ii) For the halides the problem is almost underdetermined, i.e., there is awhole set of degenerate parameters that equally well describe, e.g., chloride and bromide ions. (iii)For the heavy cations the problem is overdetermined, i.e., no combination of Lennard-Jonesparameters is able to reproduce simultaneously energy and entropy of solvation. We discuss variouspossibilities to deal with these problems and finally present an optimized force field for the halideanions that reproduces the free energy and the entropy of solvation. For the alkali metal cations thereis no unambiguous choice of parameters. Therefore, we give three different parameter sets for everyion with a small, intermediate, or large Lennard-Jones interaction strength, where the Lennard-Jonesdiameters are optimized to reproduce the solvation free energy. The ionic radius is reproduced withacceptable accuracy by this optimization strategy, meaning that the proposed force fields are reliablebeyond the target observables (i.e., free energy and entropy of solvation).