In this section, we discuss a number of technical details that are essential for making calculations of MP2 interaction energies with PWs tractable. As already mentioned, the leading computational effort for this task scales as O N 2 occ n 2 max Ω(E ρ ia cut ) 3/2 for which n max is reduced by the joint G of ρ ia (G) pair density values (Eq.6) to be stored for the two-electron integrals (Eq.7), and therefore have a strong influence on the memory requirements.