  Ideal models of complex materials must satisfy all available information
about the system. Generally, this information consists of experimental data,
information implicit to sophisticated interatomic interactions and potentially
other {\it a priori} information. By jointly imposing first-principles or
tight-binding information in conjunction with experimental data, we have
developed a method: Experimentally Constrained Molecular Relaxation (ECMR) that
uses {\it all} of the information available. We apply the method to model
medium range order in amorphous silicon using Fluctuation Electron microscopy
(FEM) data as experimental information. The paracrystalline model of medium
range order is examined, and a new model based on voids in amorphous silicon is
proposed. Our work suggests that films of amorphous silicon showing medium
range order (in FEM experiments) can be accurately represented by a continuous
random network model with inhomogeneities consisting of ordered grains and
voids dispersed in the network.
