Such laws indicate that the -3 and -4 orders are indeed good leading candidates, but that terms of higher orders may also be significant. If some rational explanation were to be found, Rovibi34 (46) is compatible with contributions resulting from the principal expansion proposed by Helgaker (power -3) and those of the highest angular momentum L present in the basis set from the partial wave expansions put forward by Caroll 80 , Hill 81 and Kutzelnigg . On the other hand, Rovibi45 (47) suggests that an additional order to Martin4 improves the extrapolation and that the (minus) third order does not dominate. For both, the X-shifts reflect not only the balance between the orders, but also between the basis functions that have L = X -1 for H and L = X for C, N and O atoms in the systems studied here. Based on these considerations, and because the leading order of Martin4 (-4) was motivated empirically by comparison with experimental atomization energies of small molecules , the Rovibi34 scheme seems more formally justified.