An iterative perturbation scheme is constructed for LEED calculations. The subplanes comprising an ideal crystal surface are assigned a definite order. and it is shown how to calculate the scattering amplitude to any order in reverse scattering. Each order involves the same computational procedure so that convergence can easily be checked. The method is similar in structure to the Renormalized Forward Scattering (RFS) theory. However. unlike RFS it can be used without supplementation by other methods when there are several inequivalent subplanes in a surface layer, and should therefore be particularly valuable for work on complex surface structures. It has great advantages of speed and low core storage requirements compared with exact methods. The scheme is tested by calculations on a Ni (111) layer, a case for which perturbation theory in the ion-core T matrices diverges, and it is found that the results of calculations to fifth order in reverse scattering are practically identical to the exact results. A convergence study shows that third-order calculations would be sufficient for most purposes.

Previous structural investigations of mesogenic organosilicon compounds (1,3-dihydroxytetraaIkyldisiloxanes, [R,(OH)Si],O, R =C,,HZn+ which form thermotropic phases are outlined. The crystal and molecular structure determination of a non-mesogenic member of this series [Me,(OH)Si],O is described. It is inferred that the mesophases formed by this family of compounds belong to a new structural type with columnar stacks of the molecules laced together with hydrogen bonds.

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