  The problem of nucleon-nucleon correlations and meson exchange currents has
been vividly debated in connection with the neutrino-nucleus cross sections. In
this work we focus on nucleon-nucleon correlations by discussing a formal
correspondence between the approaches based on independent particles and the ab
initio approaches involving correlated wave functions. We use a general
technique based on unitary transformation mapping the Fermion operators
relative to bare nucleons into quasi-particle operators relative to dressed
nucleons. We derive formulas for spectral functions, response functions,
momentum distribution, separation energy, general enough to be applied with any
kind of effective nucleon-nucleon interaction. We establish the relation
between the non-energy-weighted sum rule and the Fermi sea depopulation. With
our tools we evaluate whether approaches based on effective interactions are
compatible with the expected amount of correlations coming from ab initio
calculations. For this purpose we use as a test the Fermi sea depopulation and
the value of the kinetic energy per nucleon.
