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Aeta Cryst, Nter Reck, Leo Kutschabsky · 1979
If the atoms in a unit cell are not almost randomly distributed difficulties may arise in the normal application of direct methods. Structural information can be taken into account mainly by modification of the probablity distribution of the E values or by modification of the E values themselves. A straighforward method involving the second possibility, applied successfully to the solution of two noncentrosymmetric steroid structures, is described in this paper. The modified E values are calculated by the formula I Em(hkl)l = [ I Eobs(hkl) l 2 _ el Efrag(hkl)l 2 ]1/2 with Em(hkl) = modified E value, Eobs(hkl) = observed E value, Efrag(hkl) = theoretical E value calculated with coordinates of a correctly oriented molecular fragment.