The two components of the spectral density correspond to the intramolecular vibrational mode in the underdamped limit , ω 1 ≫ γ 1 , such that ω 1 = ω 0 and η 1 = λ, and the bath modes respectively. In the overdamped limit, where ω 2 ≪ γ 2 the second contribution is reduced to the Debye form,