Figure 13.1.3.1 

(a) A dimer in which the two subunits are related by arbitrary rotation and translation. One member of the dimer is indicated by dashed lines joining its atoms. (b) The vector set representing the Patterson function of this dimer. The intra-subunit vectors of each subunit are indicated by filled circles linked by lines; the inter-subunit vectors are shown as open circles. (c) A second copy of the Patterson function has been rotated over the original. Intra-subunit vectors of the original Patterson function are indicated by full lines; the rotated intra-subunit vectors are distinguished by dashed lines. The rotation is almost the same as the rotation of one subunit to make it parallel to the other. When the rotation is exactly correct, half of the intra-subunit vectors of one Patterson will superimpose onto the other. None of the inter-subunit vectors superimpose.