We opted for the use of DF-LCCSD(T0)/cc-pVQZ as a reference method. One particular reason for the choice is the fact that PAO-based local correlation treatments are known to significantly reduce BSSE, which is what we deem to be the major source of error in the AC(2)-QM/MM results. For the smallest systems (ammonia, carbon dioxide, cyanamide, and formamide), full optimisations including 2-body corrections at the aforementioned level were carried out. Gradients and stresses were calculated numerically, therefore requiring a full twobody calculation for each displacement structure. The results of our calculations are provided in Table and Table (full geometry cell parameters). One should once again note that symmetry is not fully exploited. In the case of CO 2 , for example, it would have been sufficient to optimise two variables (C-O bond length and a cell vector). We are keeping the same standard procedure for all systems.