In addition to a quite inexpensive DFT treatment, our model allows for the application of much more accurate QM description. Performing a AC(2)-QM/MM geometry optimisation at the LCCSD(T0)/cc-pVQZ level of theory, our method performs surprisingly well for the smallest molecules in the benchmark set. In an application to a real-life crystal structure problem our model could successfully improve results for a piano-stool complex. Its uncommon structural feature, the bending of Cp hydrogen atoms towards a cation, could not be found in gas phase calculations, but is nicely reproduced with our AC-QM/MM model.