English: Energy profiles for different hydrogen bond types between oxygen heteroatoms. Standard hydrogen bonds are asymmetrical, with the hydrogen being associated with one heteroatom. When the pKa between the heteroatoms is equal, a symmetrical hydrogen bond forms with the hydrogen in equilibrium between two locations. At shorter distances, the barrier between the two energy minima is low enough that the hydrogen is equally bound as a low-barrier, or single-well hydrogen bond.