The data set to construct the ∆V 2-b PES consists of 71 892 energies at widely dispersed geometries of the water dimer, taken from the 2-b data in q-AQUA. For each dimer structure, the differences between CCSD(T)/CBS 2-b energies and TTM3-F 2-b were computed and used as the target of the fit. We used permutationally invariant polynomials (PIPs)