**It has been shown that
a satisfying RDM model may constitute a starting model for a successful
RMC simulation with constraints.**

**The RDM best models correspond
to crystal structures in which the glasses devitrify, in all three cases.**

**One can think about what
would happen if the three partial structure factors for SiO _{2}
or ZnCl_{2} or the ten partial structure factors for NaPbM_{2}F_{9}
had been experimentally available.**

**It is not possible to
assert that the actual models proposed by either the RMC or the RDM methods
would lead necessarily to low R factors on the lacking structure factors
without any adjustment.**

**Those three dimensional
structures are simply models that are consistent with the data, constraints
and external knowledge.**

**In other words, the RDM
best model is one structure in the group of possible RMC solutions.**

**RMC tends to produce the
most disordered structure if the starting configuration is random, and
RDM produces the most ordered.**

**Combining the two methods
produces intermediate order, as expected.**

**Both methods have pro
and con.**

**Testing a model by RDM
is fast, but finding a model having the exact glass composition can be
a problem.**

**Obtaining convergence
by RMC may be quite long when drastic constraints are imposed, however,
the model size brings more credibility than for the generally small RDM
models.**

**Nevertheless, a strategy
is essential for succeeding in building models consistent with external
knowledge (no edge sharing if undesired, strict coordinations and so on).**

**Such a strategy is not
always easy to establish with the current existing RMC code, and a strategy
avoiding trigonal prisms when octahedra were exclusively required was not
found here.**

**It is expected that confidence
in RDM modelling will increase as a consequence of the present study, showing
that a good RDM model is always an excellent RMC candidate, reconciling
both methods.**

**Trying to go further in
combining both methods could be attempted.**

**The idea would be to decide
of atom moves in the RDM method by testing random displacements instead
of using the least-squares process, while still using a mean small microstrained
model.**

**Next time, may be.**

**References**

**RMC : **R.L. McGreevy and L.
Pusztai, *Molec. Simul*. **1** (1988) 359.
**RMC: **R.L. McGreevy,
in : *Computer Modelling in Inorganic Crystallography*, ed. C.R.A.
Catlow, Academic Press, London (1997) 151.
**RMC-SiO _{2}:** D.A.
Keen and R.L. McGreevy,