  The self-energy of the critical 3-dimensional O(N) model is calculated. The
analysis is performed in the context of the Non-Perturbative Renormalization
Group, by exploiting an approximation which takes into account contributions of
an infinite number of vertices. A very simple calculation yields the 2-point
function in the whole range of momenta, from the UV Gaussian regime to the
scaling one. Results are in good agreement with best estimates in the
literature for any value of N in all momenta regimes. This encourages the use
of this simple approximation procedure to calculate correlation functions at
finite momenta in other physical situations.
