description of asymptotic $NN$ scattering data [@Bal+; @Kai+].

In contrast with the OPEP, the two-pion exchange potential (TPEP) produces an attractive interaction that depends little on spin and isospin, because it is dominated by the exchange of a scalar-isoscalar system. In many phenomenological potentials, the actual two-pion dynamics is simulated by the exchange of an effective scalar-isoscalar meson, with a mass around 550 MeV. In the framework of chiral symmetry, this meson is usually identified with the counterpart of the pion in the linear sigma-model and one obtains a prediction for its coupling constant to the nucleon. The existence of a scalar-isoscalar meson is controversial, the main candidate being the f$_0$(400-1200), that may be present in $\pi\pi$ scattering [@PDG]. In the case of NN scattering, the TPEP is quite well accounted for by non-linear chiral dynamics, constrained by experimental information, and the scalar-isoscalar meson is unnecessary. According to the unwritten law of quantum mechanics, stating that processes which are not forbidden are compulsory, the exchange of two pions must be considered in any realistic description of the NN interaction and the use of the sigma field to simulate the actual TPEP gives rise to shortcomings. In particular, a scalar field 