In spite of this, expressions inspired by Eq.36 have demonstrated their potential for the extrapolation of (aug-)cc-pVXZ energies to the basis set limits, also in the case of polyatomic systems. For example, Martin et al. proposed to average between hydrogen, helium (L ∼ X -1) and first-row (L ∼ X) atoms to replace L by X -1 2 , yielding Eq. Martin4 (23) and Martin46 (24) that correspond to the leading orders found by Kutzelnigg et al. for the MP2 energies . He later suggested that the quality of the results can improve if the HF and MP2c energies are processed separately with Martinα (25) .