Calculations of potential, energy, and force terms, eqs 1-7, requires summation over MM atoms in the short-, N sr MM , and long-range, N lr MM , domains and the integration of suitable observables, involving the electronic density, or the evaluation of the electrostatic potential from the MM atoms (eq 3) to be added to the KS-DFT external potential. In a plane wave representation of the KS orbitals, the two latter operations are performed on a regular grid discretizing the space. Our basic parallel strategy consists in distributing planes of the realspace grid among the MPI processes. While the QM/MM implementation presented in this work makes use of a plane wave implementation due to the use of CPMD to compute QM contributions, MiMiC is able to support any other type of basis set that allows for a mapping onto real-space grids. A grid-distribution strategy works well as long as the number of MPI processes does not exceed the number of grid planes. To extend the scalability beyond this limit one can use two complementary approaches, task grouping and OpenMP fine-level parallelism, or a combination of the two.