Many-body computer simulation models for alkali-metal ion-water interactions

Polarizable force fields (FFs) and ab initio computer simulations methods grounded in density functional theory (DFT) can be used to describe molecular interactions. However, FFs and DFT models face a major challenge in accurately describing weak and/or hydrogen-bond interactions and how they vary from the gas to the condensed phase. This problem becomes particularly acute in computer simulations of structural, thermodynamic, dynamical, and spectroscopic properties of salt solutions. A research team at University of California in San Diego address this problem using many-body potential energy functions (MB-nrg PEFs) that are fitted to coupled cluster level theory with single, double, and perturbative triple excitations (i.e., CCSD(T)). They describe their research in The Journal of Chemical Physics.

Their MB-nrg PEFs include a detailed treatment of one-body (1B), two-body (2B), and three-body (3B) ion-water interactions. The authors describe how the MB-nrg PEFs are derived entirely from correlated electronic structure data and computationally expressed through the MB expansion. After generating training sets for the MB-nrg PEFs, the researchers analyzed 2B interaction energies for alkali-metal ion-water dimers (M⁺(H₂O) with (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺)). The resulting MB-nrg PEFs exhibit greater accuracy in comparison with existing FFs and DFT models for all five M⁺(H₂O) dimers.

Co-author and theoretical chemist Francesco Paesani states that, within the scope of simulating ion hydration, these findings demonstrate how “MB-nrg PEFs achieve higher accuracy than existing FFs and DFT models by correctly describing short-range quantum-mechanical effects associated with electron density overlap, as well as long-range electrostatic and dispersion interactions.” These findings can therefore help scientists across many disciplines answer fundamental questions, such as how alkali-metal ions regulate cell physiology and how ion hydration affects heterogeneous processes on sea-spray aerosols.

Source: “Toward chemical accuracy in the description of ion-water interactions through many-body representations. Alkali-water dimer potential energy surfaces,” by Marc Riera, Narbe Mardirossian, Pushp Bajaj, Andreas W. Götz, and Francesco Paesani, The Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4993213 .