  Hydrogen bonds are typically treated as sufficiently localized directional
intermolecular bonds, in which dispersion and electrostatic contributions can
be distinguished. However, being formed chiefly due to the overlapping of p
orbitals of electronegative atoms, the corresponding electronic bonds are
characterized by both {\sigma}- and {\pi}-kind binding, the former determining
the directionality of bonds, while the latter, the coupling of molecules and
the collective effects in H-bond networks. The latter contribution was never
considered previously and is predetermined by overlapping pre-lone pair
orbitals of oxygen atoms. This is manifested in the peculiarities of the
electron density distribution, which are quantified based on the analysis of
magnetic shielding tensors of oxygen and bridge hydrogen nuclei and illustrated
by the shapes of cluster orbitals of water aggregates.
