Neutron single-crystal diffraction is absolutely essential (1) to determine the accurate positions of hydrogen atoms in solids, and (2) to determine the arrangements of magnetic moments in solids. In many cases, hydrogen positions can be inferred from the positions of the other (carbon, oxygen, nitrogen, etc.) atoms, but if hydrogen bonding occurs, this is not reliable.
Many of the most important functions, e.g. in catalysis, pharmaceuticals and functional biology, depend on just such hydrogen bonds.
There are also applications in a wide range of materials-science problems, including the new lightweight hydrogen-storage materials (alanates, metal-organic frameworks, etc.) that are under consideration for transportation applications as a substitute for hydrocarbons.