Molecular packing groups and ab initio crystal-structure prediction

Ab initio crystal structure prediction can proceed by minimization of the packing energy of Z independent molecules per cell, or alternatively by energy minimization taking one molecule as independent and Z - 1 molecules as related by assumed space-group symmetry. In the former method, a large number of positional variables must be considered. In the latter method, a large number of space groups must be considered. An alternative, more efficient, procedure is proposed, where it is recognized that values of Z and the number of molecules in the asymmetric unit, Z', impose restrictions on possible space groups. Examples of application of this method to crystal structure prediction are given.