Within quantum chemistry, detailed investigation of noncovalent interactions in larger systems began about a decade ago with the introduction of the L7 127 and S12L test sets. The L7 complexes are depicted in Fig. , to which we have added a 132-atom buckyball-in-a-ring system (C 60 @[6]CPPA). The latter was taken from a recent study, in which several of the complexes in Fig. are found to be problematic for methods well beyond DFT, affording disagreements amongst the best ab initio benchmarks. For C 60 @[6]CPPA, guanine-cytosine on circumcoronene (C3GC), and the parallel-displaced coronene dimer (C2C2PD), benchmark interaction energies obtained from fixed-node diffusion Monte Carlo (FN-DMC) calculations disagree with CCSD(T)/ CBS results. The CCSD(T) calculations were performed using a local-orbital approximation, in this case localized natural orbital (LNO-)CCSD(T), and such approximations are not without