Vibrational relaxation is a key issue in chemical reaction dynamics in condensed phase and at the gas-surface interface, where the environment is typically highly structured and cannot be expressed in terms of a simple friction coefficient. Rather, full knowledge of the coupling of the molecular oscillator to the environment is required, as typically subsumed in the spectral density of the environmental coupling. Here, we focus on harmonic Brownian motion and investigate the effectiveness of classical, canonical position autocorrelation functions to compute the spectral density of the coupling needed to describe vibrational relaxation in complex environments. Classical dynamics is performed on model systems, and several effects are investigated in detail, notably the presence of anharmonicity, the role of a high-frequency “Debye” cutoff in the environment and the influence of the detailed structure of the latter. The spectral densities are then used in standard independent oscillator Hamiltonian models which are numerically solved at T = 0 K to investigate quantum relaxation and decoherence