Dear Giovanni,

a groundwater model shows highly non-linear behaviour under
density-dependent conditions. So if a model converges well with a
density ratio of 0, it might not be as stable with a density ratio
applied. I would recommend that you run the model for a while in
transient mode until it reaches the steady state. That might be
required for all the model runs, depending on the complexity of the
model. Density-dependent models will only converge in steady state
mode for relatively simple cases.

Good luck!

Peter