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We discuss the results of an unquenched quark model (UQM) calculation of the self energy corrections to charmonium spectrum due to the coupling to the meson-meson continuum. In our approach, the effects of quark-antiquark pairs are introduced explicitly into the QM through a 3P0 quark-antiquark pair-creation mechanism. Our results are fitted to the experimental data, so that the sum of the bare and self energies gives the values of the physical masses of the states of interest. Our results for the charmonium spectrum are used to discuss the nature of the X(3872) resonance