as illustrated in the inset of Figure . While it is well established that the MT potential requires the supercell to span at least twice the size of the density to converge DFT energies , our results show for the first time that the same criterion also applies to the MP2 energy. For practical information, all MP2 interaction energies considered in this work are converged to within 0.07 kcal/mol when setting the orthorhombic cell dimensions to r x,y,z = 1.8 times the extent of the density, measured at an 0.002 a.u. isosurface. For very high accuracy (∼0.01 kcal/mol), a ratio of 2.2 is recommended instead. Hence, Eq.43 is of significant help to ensure the recovering of the last fraction of the correlation energy, and becomes indispensable for the treatment of larger systems that would impose a too large box size and intractable computational cost.