In the particular case of the ionic embedding model, where the QM/MM boundary can be indistinct, the ESP fitting procedure was found not to be robust. New research, beyond the scope of this project, is required to improve the ESP fit algorithm or else redefine the boundary so that there is a clear distinction between the QM and MM regions. For all other systems tested, the ESP fitting procedure works well and the microiterative algorithms are fully functional and ready for use. The new methods will be made available to users in ChemShell version 3.6, which is scheduled for release in summer 2013.
Tom Keal 2013-06-04