The zero density limit of g(r) of heavy water has been calculated exploiting both ab initio and effective pair potentials. The molecular dynamic simulation and the TIP4P effective potential have been also used to reproduce the g(r) at two density states of the steam along the T=580 K isotherm. The results are compared with the experimental data at nearly the same two state points. The intensity of the experimental g(r) increases with the density while that of the computed ones decreases, Furthermore, the zero density g(r) obtained from effective pair potentials severely overestimate the experimental g(r) at the lower density, The discrepancies are very strong acid provide direct evidence of nonadditive many-body effects. Quantum mechanical studies on clusters of water molecules, available in literature, allow to give a reliable explanation of the behavior of the experimental g(r).