Fig. 3.

Decomposition of water’s PBE first-order potential ${\upsilon }_{\text{ref}}^{(1)}(\mathbf{r})$. The nonadditive potential is depicted in (a), which is the sum of the two- and three-body corrections shown in (b) and (c), respectively. The two-body correction [(b)] is the sum of individual pair-corrections $\Delta {\upsilon }_{ab}^{(2)}(\mathbf{r})$ depicted in (d), (e), and (f). The colors represent the value of the potential along the plane formed by the atoms. The x- and y-axis are expressed in Bohr and the color scale is in E_h. Note that potential in (c) is negative, whereas all other potentials are positive. The color scheme emphasizes that the magnitude of the three-body correction is so similar to the H-H pair-correction, that these potentials cancel. As a result, the nonadditivity shown in (a) is well-reproduced by the sum of (d) and (e). The solid lines are isocontour values of 0.1, 0.2, 0.3, 0.4, and 0.47 E_h