Arrays with local centers of symmetry in space groups Pca2(1) and Pna2(1)

Of the several hundred structures in the Cambridge Structural Database [ver sion 4.6 (1992), Cambridge Crystallographic Data Centre, 12 Union Road, Cam bridge, England] having space groups Pca2(1) or Pna2(1), and more than one molecule in the asymmetric unit (Z > 4), approximately three-quarters conta in local centers of symmetry. These local centers, which are not crystallog raphic centers, occur predominantly near x = 1/8, y = a in Pca2(1) or near x = 1/8, y = 0 in Pna2(1); this also holds for the limited number of exampl es we have examined of pseudo-centrosymmetric molecules with Z = 4. Local c enters at these points create unusual correlations between corresponding at oms in the two molecules.