In biochemistry and pharmacology, a ligand is generally a small molecule that forms a complex with a biomolecule to serve a biological purpose. The binding occurs by intermolecular forces, such as ionic bonds, hydrogen bonds and van der Waals forces. In general, highaffinity binding involves a longer residence time for the ligand at its receptor binding site than is the case for low-affinity binding. Highaffinity binding of ligands is often physiologically important as some of the binding energy can be used to cause a conformational change. The hydrogen bond is often described as a strong electrostatic dipole-dipole interaction, has some covalent bonding features, usually involves a limited number of interaction partners and produces interatomic distances shorter than the sum of van der Waals radius.