The Kramers problem in the energy-diffusion limited regime of very low friction is difficult to deal with analytically because of the repeated recrossings of the barrier that typically occur before an asymptotic rate constant is achieved. Thus, the transmission coefficient of particles over the potential. barrier undergoes oscillatory behavior in time before settling into a steady state. Recently, Kohen and Tanner [D. Kohen and D. J. Tanner, J. Chem. Phys. 103, 6013 (1995)] developed a method based on the phase space distribution function to calculate the transmission coefficient as a function of time in the high-friction regime. Here we formulate a parallel method for the low-friction regime. We find analytic results for the full time and temperature dependence of the transmission coefficient in this regime. Our low-friction result at long times reproduces the equilibrium result of Kramers at very low friction and extends it to higher friction and lower temperatures below the turn-over region. Our results indicate that the single most important quantity in determining the entire time evolution of the transmission coefficient is the rate of energy loss of a particle that starts above the barrier. We test our results, as well as those of Kohen and Tanner for the Kramers problem, against detailed numerical simulations.