Hydrogen bonding (H-bond): A noncovalent (van der Waals) attractive force caused by electrostatic attraction of a hydrogen atom with a lone pair of another atom. The hydrogen bond donor must have a sufficiently large δ⁺ charge caused by bonding to a highly electronegative element (O, N, or F; or in uncommon cases by strong electron-withdrawing inductive effects). The hydrogen bond acceptor must have a lone pair, and sufficiently high electron density (the accepting atom must have a negative formal charge, or if neutral must be oxygen or nitrogen).