Equilibrium contact probabilities in dense plasmas

Nuclear reaction rates in plasmas depend on the overlap (contact) probability of the reacting ions. Path-integral Monte Carlo calculations are used here to determine these contact probabilities, $g(0)$, for one-component plasma (OCP) with emphasis on many-body quantum effects which can lead to order of magnitude changes. An intuitive explanation for these effects is presented. The small r behavior of $g(r)$ for quantum systems and the relation to free energies is then derived and compared to the path-integral results. Going beyond the uniform background approximation, electron screening effects and the limits of the ``constant energy shift'' approximation are discussed. Thermodynamic properties for the quantum OCP are analyzed in a final section.