Diffusion Monte Carlo calculations of D 0 for the dimer and trimer Unconstrained diffusion Monte Carlo (DMC) calculations were preformed for the zero-point energies of the water dimer and trimer using the q-AQUA-pol. Recall that these are rigorous "exact" quantum calculations and give the exact dissociation energy, D 0 , when combined with the electronic dissociation energy (D e ) and exact ZPE of the water monomer. Table 1 shows the electronic dissociation energy (D e ), ZPEs of the reactants and products, and ZPE-corrected dissociation energy (D 0 ) for the following dissociations: (