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.
).