In this work we have used the full reaction scheme in the calculation of the surface acidities developed previously, as illustrated in Fig. . Deprotonation of Bi 7f OH 2 group was coupled to a simultaneous protonation of H 2 O in the bulk electrolyte of the same inhomogeneous model system (Reaction S1 †); similarly, the deprotonation process of a BiVO br H + group was accompanied by another protonation of H 2 O in the bulk electrolyte (Reaction S2 †), the reverse of this reaction indicates a bridging oxygen site receiving a proton from the acidic bulk electrolyte. The corresponding deprotonation free energy ∆A 1 and ∆A 2 were calculated from a coupling parameter integral of the corresponding vertical energy gap (see Fig. and S2 †). The pK a values we found for the two active surface hydroxyl groups are 6.2 and 0.8, respectively, giving the PZC value of 3.5 (Table ), in excellent agreement with the experimental characterization 2.5-4.5 and previous theoretical works . Since H-bond network have a significant effect on pK a values, both the coordination numbers and the length of H-bond can lead to a vital variation in the picture of the surface acidity. To further interpret the effect of water adsorption energy E ads on pK a shifts, the RDFs between oxygen atoms of surface groups and hydrogen atoms in electrolyte are shown in Fig. . The comparison of H-bond coordination numbers of BiVO 4 with that of TiO 2 (110) and SnO 2 (110) surfaces are shown in Table . Note that the Hbond coordination numbers are obtained by integrating the first peaks of RDFs in Fig. , and the length of H-bond for oxygen atoms on BiVO 4 surface is determined by the corresponding position of the first peak. Because of the weakest interaction between the adsorbed water molecules and BiVO 4 surface sites, the interaction between adsorbed water and hydrogen in electrolyte should increase. It is indeed found a higher H-bond coordination number 0.97 for Bi 7f OH 2 group than that of TiOH 2 (0.6) and SnOH 2 (0.3) groups. On the one hand, Bi 7f OH 2 site forming more H-bonds with solvent water makes it more stable, leading the value of pK a1 larger than SnO 2 . On the other hand, the length of H-bond of Bi 7f OH 2 (blue dashed line in Fig. ) is significantly larger than Bi 7f OH -(blue line in Fig. ), while the length of Hbond of TiOH 2 is the same as TiOH . That makes the value of pK a1 on BiVO 4 surface smaller than TiO 2 .