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.