such that, for a computer time associated with MP2 that scales as N 5 N 4 b where N is the number of atoms, improving the correlation energy with a larger basis grows as N 5 X 12 . Q, 5 or 6 zeta calculations may therefore be prohibitively expensive for larger systems of interest . This is in contrast to PWs, for which the basis size does not depend explicitly on the number of atoms N but only on the volume of the supercell and the cutoff energy, so that the MP2 energy scales as N 2 occ n 2 max Ω(E