Atomic correlations and van der Waals forces

The effective potential between two electron-proton pairs at distance r in thermal equilibrium with a weakly coupled classical plasma is calculated and shown to decay as r(-6) for all temperatures and densities. In the atomic regime (low temperature and low density), it reduces to the usual van der Waals potential computed for hydrogen atoms in their ground states. In the limit of full ionization (high temperature), it is given by the r(-6)-correlation that also exists between unbound charges. The model demonstrates that the conventional van der Waals forces and the algebraic tails generally found in the quantum Coulombic correlations have the same common origin in the intrinsic quantum fluctuations of the charges.