A theoretical formula for single-atom diffusion rates that predicts an isothermal correlation relation between the liquid (l) and gas (g) phase diffusion coefficients, D(T, rho(i)) and D(T, rho(g)) is developed. This formula is based on a molecular level expression for the atom's diffusion coefficient, D(T, rho), and on numerical results for 1715 thermodynamic states of 25 rare gas fluids. These numerical results show that at fixed temperature, T, the decay time, tau(DIF), which governs the shortest time decay of an appropriate force autocorrelation function, < F(t) F >(0), is density (rho)-independent. This independence holds since tau(DIF) arises from the rho-indepenclent shortest time inertial motions of the solvent. The rho independence implies the following 1-g diffusion coefficient correlation equation: D-1(T, rho(l))= (rho(l)/rho(g)) D-1(T, rho(g)) [rho(-1)(l) < F-0,l(2)>/rho g(-1)< F(0,)g(2)>] This relation is identical in form to the familiar (isolated binary-collision-like) empirical correlation formula for vibrational energy relaxation rate constants. This is because both correlation relations arise from inertial dynamics. Inertial dynamics always determines short-time fluid motions, so it is likely that similar correlation relations occur for all liquid phase chemical processes. These correlation relations will be most valuable for phenomena dominated by short time scale dynamics.