The use of correctly oriented molecular fragments for a modification of E values

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 probability distribution of the E values or by modification of the E values themselves. A straightforward 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 |E_m(hkl)| = [|E_obs(hkl)|² - c|E_frag(hkl)|² ]^1/2 with E_m(hkl) = modified E value, E_obs(hkl) = observed E value, E_frag(hkl) = theoretical E value calculated with coordinates of a correctly oriented molecular fragment.