polarization observables at low energy. In the following we analyze the URIX potential which has two parameters, called A PW 2 π and A$_{R}$ . In this model the strength of the d -term is related to the strength of the b -term as d = b/ 4. The original values of the parameters have been fixed in Ref. [3] in conjunction with the AV18 NN potential and, from Table I, we observe that the model correctly describes the triton binding energy. However, it overestimates B ( $^{4}$He) and underestimates $^{2}$a$_{nd}$ . In order to further analyze the origin of this behavior, we have varied the constants A PW 2 π , A$_{R}$ and the relative strength D PW 2 π = d/b of the b - and d -terms. The regularization parameter has been held fixed at its original value, c = 2 . 1 fm − $^{2}$. For a given value of A PW 2 π , the values of A$_{R}$ and D PW 2 π has been chosen to reproduce B ( $^{3}$H) and $^{2}$a$_{nd}$ . The results are shown in Fig. 4. In panel (a), D PW 2 π is given as a function of A PW 2 π with A$_{R}$ varying from 0 . 0176 MeV at A PW 2 π = − 0 . 02 MeV to 0 . 0210 MeV at A PW 2 π = − 0 . 050 MeV. These values of A$_{R}$ are more than three times bigger than the original value of 0 . 0048 MeV. In panel (b) and (c) the results for $^{2}$a$_{nd}$ and B ( $^{4}$He) are given respectively. The latter has not been included in the determination of the parameters, since D PW 2 π and A$_{R}$ have been determined from the triton binding energy and $^{2}$a$_{nd}$ , and is therefore a pure prediction. We observe a slightly overestimation of B ( $^{4}$He), in particular for values of | A PW 2 π | > 0 . 04 MeV, corresponding to values of D PW 2 π < 0 . 7. B. Urbana IX Force With modifications of the parameters in the URIX force, we were able to describe rea- sonably well B ( $^{3}$H), $^{2}$a$_{nd}$ and B ( $^{4}$He). However, this has been achieved with a substantial increase of the repulsive term. In order to gain insight on the consequence of the new parametrizations in the quantities of interest, in Table III we report the mean values of the kinetic energy and the NN potential energy as well as the mean values of the attractive part of the TNF, V$_{A}$ (3 N ), corresponding to the sum of the b and d -terms, and the repulsive part, V$_{R}$ (3 N ), corresponding to the A$_{R}$ term, for selected values of the parameters (indicated as points in Fig. 4). The last two columns show B ( $^{4}$He) and $^{2}$a$_{nd}$ . For the sake of comparison, in the first row, the values obtained using the original AV18+URIX model are reported. From the table we observe that some of the values considered for D PW 2 π and A$_{R}$ are quite far from the original ones. At the original value of A PW 2 π , − 0 . 0293 MeV, the relative strength 11