A field of chemical research concerned with the formation and properties of large assemblies of molecules held together by intermolecular forces (hydrogen bonds, van der Waals’ forces, etc.). One feature of supramolecular chemistry is that of self-assembly (see self-organization), in which the structure forms spontaneously as a consequence of the nature of the molecules. The molecular units are sometimes known as synthons. Another aspect is the study of very large molecules able to be used in complex chemical reactions in a fashion similar to, for example, the actions of the naturally occurring haemoglobin and nucleic acid molecules. Such molecules have great potential in such areas as medicine, electronics, and optics. The helicate and texaphyrin molecules and dendrimers are typical examples of compounds of interest in this field. The field also includes host-guest chemistry, which is concerned with molecules specifically designed to accept other molecules. Examples include crown ethers, cryptands, and calixarenes.