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The contributions of some of the strongly coupled collective excitations in 12C + ~2C to the heavy ion optical potential have been explicitly obtained by calculating the exact Green's function for this problem. The resulting nonlocal heavy ion potential and the trivially equivalent local potential are calculated and their J-and E-dependence are discussed. It is shown that the strong direct excitations contribute significantly to the absorptive part of the heavy ion potential in the surface region.There exist now many theoretical attempts to calculate the real parts of the heavy ion potential [1,2]. These are mainly based on the folding model [-1, 2] but some of them also include the polarization effects in an adiabatic description [2]. All of these calculations are moderately successful. In particular the folding model has been shown to be able to account for many elastic scattering data with only small renormalizations [1]. This is not the case for the imaginary part of the heavy ion potentials. Here no successful and reliable calculations exist so far. Only very recently attempts have been made to calculate the complete HI potential [3-5] e.g. by determining an effective G-matrix in nuclear matter and then applying a local density approximation [4,5]. These calculations, however~ yield significantly too little absorption [-5], probably due to the fact that they neglect the strong surface * Work supported by GSI Darmstadt, the BMFT and the U.S. D.O.E. ** Permanent address: Institut ftir Theoretische Physik, Universitat Giessen, D-6300 Giessen, FRG interactions that are believed to absorb a large part of the incoming flux in heavy ion reactions [6]. In this paper we exploit just this property by calculating the contributions of strong low lying collective states to the heavy ion potential. Since most of the remaining flux at tow energies goes into fusion we introduce an additional smooth absorptive potential of Woods Saxon type to describe the contributions from the compound nuclear processes. The calculations are performed by evaluating the exact Green's function for the space of explicitly included states. With this propagator Feshbach's expression [7] for the heavy ion potential is calculated. Thus real and imaginary polarization potential are treated consistently on the same basis. In Feshbach's projection formalism the HI-potential is given by [7]:Vop, = G~, + Go( E -Hoe + i ~)-i VQ~, = Gp + d V:.The second term is the polarization potential. P and Q project on the elastic and inelastic channels, re-
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