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 [18]. e.g.
where 
is a weighting factor assigned to the or and is dependent on the error in the
calculation of (see Sec. 
).