The Kovacs hump described above has been rationalized by employing linear response theory for molecular systems, in which the initial and final states are equilibrium ones. Therein, the "direct" relaxation function (with only one temperature jump, instead of two) is a superposition of positive exponentially decaying modes, as a consequence of the fluctuation-dissipation theorem. Linear response makes it possible to write the Kovacs hump in terms of the direct relaxation function. Specifically, the positivity of the all the modes in the direct relaxation function ensures the "normal" character of the hump, i.e. the fact that Δ Δ P ≥ ≥ 0 .