A simple method for locating unoccupied spaces in a proposed crystal structure

The presence of unoccupied space of sufficient cross section lying outside the van der Waals surfaces of the atoms in a crystal structure may mean that atoms have been omitted from the structure. A simple graphical method has been devised to display these voids. When such voids occur because of the omission of atoms from a trial structure, the significance of peaks in the difference map in the region of the void is enhanced. This makes it possible to detect and, within limits, to place poorly resolved atoms.