Link Atom:

The simplest way is to use a link-atom pseudopotential. In the simplest case, this would be a scaled down pseudopotential with the required valence change (e.g. ZV=1 when cutting through a single carbon-carbon bond). However in this case it is required to constrain the distance between the link atom and the (full-QM) neighbor atom to the (full-QM) equilibrium distance, to preserve the electronic structure in the center of the QM subsystem. You should be aware of the fact, that this is a rather crude approximation and that the constraint will create a small imbalance in the forces between the QM and MM subsystems, that can result in a drift in the total energy, if the length of the constraint is badly chosen.

A more rigorous approach would be to use an optimized pseudopotential constructed with the method described in ref. [132], that should take care of the need for the constraint.