The classical method of MAD phasing is similar to that described in Sec. 
except
that there is no native structure factor 
and there is additional information present
due to the anomalous scattering. The change in heavy atom scattering factor 
due to a change in X-ray
energy is exactly analogous to, but in general smaller than, the isomorphous change affected by the
addition of a heavy atom into the
structure in the isomorphous replacement method and may be treated as such. The 
scattering factor however acts orthogonally to and is treated slightly differently.
The method is an extension of Single Isomorphous Replacement with Anomalous Scattering
(SIRAS) [88] [66].
The lack of native protein structure factor is accommodated by choosing a structure factor at one of the X-ray
energies as a reference. The reference structure factor 
is defined in Fig. and
its magnitude is given by an expression analogous to Eq. , i.e.
The phase of the reference structure factor is given by Eq. and in terms of the present variable
system for structure factors measured at two energies 
and 
this may be written
where
A second expression for 
may be obtained by rearranging Eq. to give
Again Eqs. and together produce an unambiguous solution for the phase
of .
