The cutoff energy E ϕ cut of the wavefunction has been set to 150 Ry, and the density cutoff to the usual E ρ cut = 4E ϕ cut for all systems and supercell sizes. No change larger than ∼ 0.01 kcal/mol was observed on the extrapolated MP2 interaction energies at larger cutoffs (cf. Table in the Supplementary Material). The effects of the cutoff energy for the MP2c pair densities, the supercell dimensions as well as the decoupling between periodic images are discussed below in Section IV A.