  We report here the development of hybrid quantum mechanics/molecular
mechanics (QM/MM) interface between the plane-wave density functional theory
based CPMD code and the empirical force-field based GULP code for modeling
periodic solids and surfaces. The hybrid QM/MM interface is based on the
electrostatic coupling between QM and MM regions. The interface is designed for
carrying out full relaxation of all the QM and MM atoms during geometry
optimizations and molecular dynamics simulations, including the boundary atoms.
Both Born-Oppenheimer and Car-Parrinello molecular dynamics schemes are enabled
for the QM part during the QM/MM calculations. This interface has the advantage
of parallelization of both the programs such that the QM and MM force
evaluations can be carried out in parallel in order to model large systems. The
interface program is first validated for total energy conservation and parallel
scaling performance is benchmarked. Oxygen vacancy in {\alpha}-cristobalite is
then studied in detail and the results are compared with a fully QM calculation
and experimental data. Subsequently, we use our implementation to investigate
the structure of rhodium cluster (Rh$_n$ ; $n$=2 to 6) formed from
Rh(C$_2$H$_4$)$_2$ complex adsorbed within a cavity of Y-zeolite in a reducible
atmosphere of H$_2$ gas.
