The crystal structure of alloxan ((CO)(3).NH.CO.NH) is unusual in that
it contains no hydrogen bands, We account for this by using a realist
ic model for the intermolecular forces which includes a distributed mu
ltipole electrostatic model, to find minimum energy dimer and crystal
structures. Alloxan dimer structures with close (C=O)...(C=O) interact
ions are predicted to have similar energies to the hydrogen bonded dim
ers. An extensive systematic search for possible crystal structures fo
r alloxan found the observed crystal packing to have the lowest lattic
e energy and hydrogen bonded polymorphs to be less stable by more than
5 kT mol(-1). This illustrates the limitations of the functional grou
p approach to predicting molecular crystal structures.