hydrogen bond in theoretical organic chemistry

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 315 kcal mol (1265 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 MOnY of the hydrogen acceptor molecule.
PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1945