  Predicting the properties of neutron-rich nuclei far from the valley of
stability is one of the major challenges of modern nuclear theory. In heavy and
superheavy nuclei, a difference of only a few neutrons is sufficient to change
the dominant fission mode. A theoretical approach capable of predicting such
rapid transitions for neutron-rich systems would be a valuable tool to better
understand r-process nucleosynthesis or the decay of super-heavy elements. In
this work, we investigate for the first time the transition from asymmetric to
symmetric fission through the calculation of primary fission yields with the
time-dependent generator coordinate method (TDGCM). We choose here the
transition in neutron-rich Fermium isotopes, which was the first to be observed
experimentally in the late seventies and is often used as a benchmark for
theoretical studies. We compute the primary fission fragment mass and charge
yields for 254 Fm, 256 Fm and 258 Fm from the TDGCM under the Gaussian overlap
approximation. The static part of the calculation (generation of a potential
energy surface) consists in a series of constrained Hartree-Fock-Bogoliubov
calculations based on the D1S, D1M or D1N parameterizations of the Gogny
effective interaction in a two-center harmonic oscillator basis. The
2-dimensional dynamics in the collective space spanned by the quadrupole and
octupole moments is then computed with the finite element solver FELIX-2.0. The
available experimental data and the TDGCM post-dictions are consistent and
agree especially on the position in the Fermium isotopic chain at which the
transition occurs. The main limitation of the method lies in the presence of
discontinuities in the 2-dimensional manifold of generator states.
