  Hydrogen bond (H-bond) covalency has recently been observed in ice and liquid
water, while the penetrating molecular orbitals (MOs) in the H-bond region of
most typical water dimer system, (H2O)2, have also been discovered. However,
obtaining the quantitative contribution of these MOs to the H-bond interaction
is still problematic. In this work, we introduced the orbital-resolved electron
density projected integral (EDPI) along the H-bond to approach this problem.
The calculations show that, surprisingly, the electronic occupied orbital
(HOMO-4) of (H2O)2 accounts for about 40% of the electron density at the bond
critical point. Moreover, the charge transfer analysis visualizes the electron
accumulating effect of the orbital interaction within the H-bond between water
molecules, supporting its covalent-like character. Our work expands the
classical understanding of H-bond with specific contributions from certain MOs,
and will also advance further research into such covalency and offer
quantitative electronic structure insights into intermolecular systems.
