Binding energies of the water cluster vs the number of water molecules. Black line, results from HF/6-31G(d,p) calculations; red line, SCCDFTB energies with the original γ function; green line, SCCDFTB energies with the modified γ function for the H-bond; blue plus, SCCDFTB energies with the modified γ function for a set of DFT optimized structures. 45 For the latter three, the structures were re-minimized with the corresponding SCCDFTB methods prior to the calculation of energies.