The described QM/MM methodology has been implemented using a plane wave (PW) basis where the computationally most expensive part is the calculation of the potential and forces originating from the short-range interactions which require integration of the electron density over the entire real space grid for each MM atom in the short-range part. The corresponding long-range interactions involve only one integration of the electron density, which is performed when determining the QM multipoles, and can thus lead to a substantially reduced computational cost compared to the full explicit coupling used for the short-range part. In section 5, we investigate the effect of the long-range coupling with respect to the short-range cutoff distance and the order of the multipole expansion in detail.