Fig. 1.

Two views of the same configuration of the water dimer, with atoms marked by numerical labels used throughout this article. The relative orientation of the monomers is the same as that of the global energy minimum, which contains a mirror plane and a single hydrogen bond. Topological atoms are bounded by interatomic surfaces at the system’s inside and by envelopes of constant electron density at the outside. The bond critical points (which are at the respective centres of the interatomic surfaces) are marked by small purple balls. These images were generated by the in-house program [56] IRIS, which is based on a finite element algorithm [57]