Nucleon transfer is studied with a focus on charge equilibration. It is completely classified into six types.
The preferred nucleon transfer (i.e. neutron transfer, proton transfer, dual-way transfer, etc.) is predictable
based on this classification, which is verified within the framework of microscopic time-dependent density
functional calculations. This classification gives a simple explanation for the reason why the proton-richness
does not necessarily result in the enhancement of proton transfer contrary to the neutron-richness in neutron