water molecule acts as a donor or acceptor (or both). As expected, the shifts in IE (δ) are mostly positive when the added water molecule acts as hydrogen-bond donor and negative when the added water molecule acts as hydrogen-bond acceptor. For the structures used to construct Fig. , the δIE for hydrogen-bond acceptor structures, hydrogen-bond donor structures, and non-hydrogen bonded waters range between -0.0161 and -0.15 eV, -0.024 and 0.1086 eV and -0.0315 and 0.0388 eV, respectively. The waters that act as both hydrogen bond acceptor and donor result in δ between 0 and -0. The above analysis shows that waters that are hydrogen-bonded to the first solvation shell have the strongest effect on the IE of the central water. Hence, we can use this criterion of building up the QM system instead of the distance from the central water. Fig. shows the convergence of the core IE of the central water with respect to the QM system size using these two criteria for growing the QM system. The smallest QM comprises 5 water molecules (we pick a water molecule at the center of the simulation box and chose 4 closest waters).