The program CRYSTALS [Betteridge, Carruthers, Cooper, Prout & Watkin (2003). J. Appl. Cryst. 36, 1487] has been extended to include an option for the re®nement of a continuous electron density distribution lying along a line, a ring or on the surface of a sphere. These additional non-atomic electron distributions can be re®ned in any combination with traditional anisotropically distributed spherical atoms, including the re®nement of`partial' atoms overlapping with the special electron distributions.

No abstract