PNNL

Abstract

We present a classical induction model to evaluate the 3-body Ion-Water-Water (I-W-W) and (W-W-W) interactions in aqueous ionic systems. The classical description of the induction energy is based on electrostatic distributed multipoles up to hexadecapole and distributed polarizabilities up to quadrupole–quadrupole on the O and H atoms of water. The monatomic ions were described by a point charge and a dipole-dipole polarizability, while for the polyatomic ions distributed multipoles up to hexadecapole and distributed polarizabilities up to quadrupole–quadrupole were used. The accuracy of the classical model is benchmarked against an accurate dataset of 936 (I-W-W) and 2,184 (W-W-W) 3-body terms for 13 different monatomic and polyatomic cation and anion systems. The classical model shows excellent agreement with the reference MP2 and CCSD(T) 3-body energies. The Root-Mean-Square-Errors (RMSEs) for monatomic cations, monatomic anions, and polyatomic ions were 0.29 kcal/mol, 0.25 kcal/mol, and 0.12 kcal/mol, respectively. The corresponding RMSE for 1,744 CCSD(T)/aVTZ 3-body (W-W-W) energies, used to train MB-pol, was 0.12 kcal/mol. The accuracy of the classical model demonstrates that the 3-body term for aqueous ionic systems can be accurately modeled classically, without the need to fit to tens of thousands of high-level ab initio calculations. This approach provides a fast, yet accurate and efficient path towards modeling the 3-body effect in aqueous ionic systems that is fully transferable across systems with different ions.