  The approximate representation of a quantum solid as an equivalent composite
semi-classical solid is considered for insulating materials. The composite is
comprised of point ions moving on a potential energy surface. In the classical
bulk domain this potential energy is represented by pair potentials constructed
to give the same structure and elastic properties as the underlying quantum
solid. In a small local quantum domain the potential is determined from a
detailed quantum calculation of the electronic structure. The primary new
ingredients are 1) a determination of the pair potential from quantum data for
equilibrium and strained structures, 2) development of pseudo-atoms for a
realistic treatment of charge densities where bonds have been broken to define
the quantum domain, and 3) inclusion of polarization effects on the quantum
domain due to its environment. This formal structure is illustrated in detail
for an silica nanorod. For each configuration considered, the charge density of
the entire solid is calculated quantum mechanically to provide the reference by
which to judge the accuracy of the modeling.It is then shown that the quantum
rod, the rod constructed from the classical pair potentials, and the composite
classical/quantum rod all have the same equilibrium structure and response to
elastic strain. The accuracy of the modeling is shown to apply for two quite
different quantum chemical methods for the underlying quantum mechanics:
transfer Hamiltonian and density functional methods.
