Have a functioning QM/MM code - which will soon be available in the development version. It produces reasonable radial distribution functions btw. classical water and quantum water, and quantum ions, when compared to experiment and other similar codes.
As the code is written now it assumes that the quantum region is completely screened by the classical environment (no pbc), and that there is enough classical stuff (which is cheap) such that the classical interaction btw. supercells can be treated with the minimum image convention.
Currently working on:
a sensible way to optimize Lennard-Jones parameters for the QM system such that QM-MM interactions are as close as possible to QM-QM interactions (seems to be the norm in literature)
an algorithm to fit classical inter- and intrapotentials to QM results (i.e. MM parameters on the run)
Quasi-Newton for QM/MM systems.