Fragment Quantum Mechanical Method for Large-Sized Ion-Water Clusters

Fragmentation methods have been widely studied for computing quantum mechanical (QM) energy of medium-sized water clusters, but less attention has been paid to large-sized ion-water clusters, in which many-body QM interaction is more significant, because of the charge-transfer effect between ions and water molecules. In this study, we utilized electrostatically embedded generalized molecular fractionation (EE-GMF) method for full QM calculation of the large-sized ion-water clusters (up to 15 Na⁺ and 15 Cl^- ions solvated with 119 water molecules). Through systematic validation using different fragment sizes, we show that, by using distance thresholds of 6 Å for both the two-body and three-body QM interactions, the EE-GMF method is capable of providing accurate ground-state energies of large-sized ion-water clusters at different ab initio levels (including HF, B3LYP, M06-2X, and MP2) with significantly reduced computational cost. The deviations of EE-GMF from full system calculations are within a few kcal/mol. The result clearly shows that the calculated energies of the ion-water clusters using EE-GMF are close to converge after the distance thresholds are larger than 6 Å for both the two-body and three-body QM interactions. This study underscores the importance of the three-body interactions in ion-water clusters. The EE-GMF method can also accurately reproduce the relative energy profiles of the ion-water clusters.