Given an initial set of phases , from a refined model for example, or from initial phases calculated by isomorphous replacement, the heavy atom positions in a subsequent derivative structure may be determined by calculating a difference Fourier, i.e. a Fourier synthesis using isomorphous or Bijvoet differences as coefficients to produce difference electron density corresponding approximately to the partial structure of substituted atoms . e.g.
where is a weighting factor assigned to the or and is dependent on the error in the calculation of (see Sec. ).