UC San Diego

Water is ubiquitous in nature. The arrangement and behavior of water molecules around

solutes, ranging from monoatomic ions such as alkali metal ions, to small organic molecules or

proteins, is important to understand the driving forces of many processes and reactions. Computer

simulations have become a powerful tool within the last decades, but a unified molecular level

picture of the mechanism of hydration is still missing. On one hand, ab initio methods are

accurate, but the treatable size of the system is relatively small, and their accuracy depends on the

level of theory used. On the other hand, force fields allow for the study of large system sizes, but

at the cost of accuracy. In this work, we present another approach, the MB-nrg (for many-body

energy) potential energy functions (PEFs), which have the speed of a force field and the accuracy

of 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 the

MB-nrg framework, many-body effects are described through classical polarization, and explicit

corrections 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, it

is shown that adding higher levels of corrections to the classical polarizable model improves

the agreement between experiments and our theoretical predictions. Finally, this methodology

is extendable to any insulator, i.e. the electrons are not delocalized over the whole system, for

which accurate reference energies can be obtained. However, the software infrastructure available

right 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) that

enables efficient energy calculations. The combination of MBX with MB-nrg is a powerful

tool that allows us to study any complex system, as long as accurate reference energies can be

obtained.