Understanding Many-Body Effects in Ion Hydration with the MB-nrg Potential Energy Functions

Water is ubiquitous in nature. The arrangement and behavior of water molecules aroundsolutes, ranging from monoatomic ions such as alkali metal ions, to small organic molecules orproteins, is important to understand the driving forces of many processes and reactions. Computersimulations have become a powerful tool within the last decades, but a unified molecular levelpicture of the mechanism of hydration is still missing. On one hand, ab initio methods areaccurate, but the treatable size of the system is relatively small, and their accuracy depends on thelevel of theory used. On the other hand, force fields allow for the study of large system sizes, butat the cost of accuracy. In this work, we present another approach, the MB-nrg (for many-bodyenergy) potential energy functions (PEFs), which have the speed of a force field and the accuracyof the current “gold standard” in electronic structure calculations: coupled-cluster with singles,doubles and perturbative triples excitations (CCSD(T)) at the complete basis set limit. In theMB-nrg framework, many-body effects are described through classical polarization, and explicitcorrections using permutationally invariant polynomials (PIPs) fitted to reproduce CCSD(T)reference data are added at the two- and three-body level. Focusing on the alkali metal ions, itis shown that adding higher levels of corrections to the classical polarizable model improvesthe agreement between experiments and our theoretical predictions. Finally, this methodologyis extendable to any insulator, i.e. the electrons are not delocalized over the whole system, forwhich accurate reference energies can be obtained. However, the software infrastructure availableright now does not allow the use of this kind of methodology in an efficient way. Consequently,we have also developed a new software infrastructure MBX (for many-body expansion) thatenables efficient energy calculations. The combination of MBX with MB-nrg is a powerfultool that allows us to study any complex system, as long as accurate reference energies can beobtained