In 1999, Isaacs et al. proved the above. Hydrogen bond was most generally conceived as electrostatic interaction between e.g. OH and O (this doesn't define h-bond as partial covalent just as it is partial ionic), as electron clouds on Hydrogen is biased towards the electronegative O and this creates a partial proton that interact electrostatically with the opposite O. How did they come to conclude that the interaction between (OH as one entity) and (O) is partial covalent?