A particular type of multicenter (three center - four electron) X–H ...Y in which the central hydrogen atom covalently linked to an electronegative atom X (C, N, O, S...) forms an additional weaker bond with atom Y (N, O, S...) in the direction of its lone electron pair orbital. The energy of hydrogen bonds, which is usually in the range of 3-15 kcal/mol (12-65 kJ/mol), results from the electrostatic interaction and also from the orbital interaction of the antibonding σ*(XH)MO of the molecule acting as the hydrogen donor and the non-bonding lone electron pair MOn_Y of the hydrogen acceptor molecule.