Molecular clusters with one symmetry operator

Translation of an I nucleus leads to space group P1,Z = 2, (see Fig. 5.2). For an Sy cluster (a screw axis with translation along y), the variables are the distance between the center of mass and the screw axis, the screw pitch, and three molecular 

Symmetry Variable orientation Variable coordinates Resulting translation elements Resulting structure 

To introduce centering, a search is conducted on two possible orthogonal translations (x,y) of the original molecular object, the other three variables being, as usual, the molecular orientation angles a layer is obtained. In any crystal structure resulting from these layers, the a cell parameter must be equal to twice the x translation, and b to twice the y translation. 

The only way of dealing with multimolecular asymmetric units in this approach is to provide an optimized structure of the symmetry-less dimer, and to use this dimer as the repetition unit throughout the procedure. The total lattice energy is then the sum of the lattice energy of the dimer and of the dimer cohesion energy (Section 8.7). 

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Table 14 A A summary of procedures for the search molecular clusters with one symmetry operator see also Fig. 14.8...

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