Figs. and show the MPAC quantities for H - and Ne 8+ from the He isoelectronic series (Z = 1 and Z = 10). Panel (a) shows the global MPAC integrand E λ c (Eqs. 3 and 4) and its MP2 linear approximation, 2E MP2 c λ (Eq. 5). As already noted in Ref. c becomes increasingly linear as Z increases for the He isoelectronic series, indicating stronger curvature and higher-order correlation (beyond dynamical) effects in H -(Fig. ) compared to Ne 8+ (Fig. ).