By itself this change will produce a much worse description of A$_{y}$ and iT 11 . However this is well compensated with the corresponding increase in $^{4}$P 5 / 2 and ϵ 3 / 2 − . This is not the case with the minimum in T 21 , for which a better description would be obtained lowering the AV18 value of $^{4}$P 1 / 2 , as discussed in Ref. [26]. The other parametrizations given in Tables II, III, IV produce similar changes in the $^{4}$P$_{J}$ parameters. From this observation we can conclude that the spin-isospin structure of the TNFs considered here is not sufficient to describe simultaneously B ( $^{3}$H), $^{2}$a$_{nd}$ , B ( $^{4}$He) and the vector and tensor analyzing powers at low energies. V. CONCLUSIONS Stimulated by the fact that some of the widely used TNF models do not reproduce simultaneously the triton and the $^{4}$He binding energies and the n − d doublet scattering length, we have analyzed possible modifications to their parametrizations. To this end we have selected the AV18 as the reference two-nucleon force and, associated with it, we have varied the original parameters of the TM’ and URIX models so as to improve the description of the three quantities mentioned. Furthermore, using the recent local form of a chiral TNF (we have called this model N2LOL), we have studied its parametrization associated to the AV18 interaction too. The analysis has proceeded in the following way. The three models under observation, TM’, AV18 and N2LOL, have been written in configuration space as a sum of five terms, the a -, b -, d -, C - and E -terms. The first three, corresponding to a two- pion exchange process, are attractive. The last two, corresponding to contact terms, can be either attractive or repulsive. Not all the models include the five terms. In the TM’ model only the a -, b - and d -terms are present and therefore, this model does not include explicitly a repulsive term. The URIX model includes the b -, d - and E -terms. This last term has been parametrized as repulsive in order to compensate the large overbinding produced by the first two terms in infinite nuclear matter. The N2LOL model includes the five terms. The study has been started analyzing the AV18+TM’ model. Maintaining fixed the strength of the a -term at its original value, we have varied the strengths of the b - and d - terms for several values of the cutoff parameter Λ. We have explored negative values of the strength parameters b and d in order to keep the attractive character of these terms. We have found it difficult to reproduce $^{2}$a$_{nd}$ for reasonable values of the strength parameters. 16