As Fig. shows, increasing the QM size results in a red shift of the 1s O IE of the central water. The minimal QM is clearly not sufficient-increasing the size from 5 to 20 waters leads to the red shift of 0.3-0.4 eV. The convergence is not monotonous and quite slow-we observe small fluctuations (∼0.1 eV) even beyond 25 waters. Fig. also shows that selecting waters by the distance from the first solvation shell is more effective and results in faster and smoother convergence. For the QM size of 20 water molecules, the difference in the IE between protocol 1 and 2 is 0.06 eV and between protocol 2 and 3 (basis-set effects)-0.107 eV. We note that the difference due to using a smaller basis increases with the system size (5-12) and then becomes nearly constant (about 0.10 eV for the QM sizes of 12-20). Hence, in our production-level simulations we use protocol 3 and estimate the basis-set correction by taking a difference between protocols 2 and 3 for a small number of snapshots. Figure shows the convergence of the shift in 1s O IE of the central water for the simulation with the 20 QM waters between protocol 2 and protocol 3. The average value of the shift (δ basis ) is 0.105 eV. The estimated shift δ basis = 0.105 eV, with a 0.001 eV standard deviation.