In practice, because quantum correlation functions are usually difficult to calculate, classical autocorrelation functions obtained from classical simulations are more often used for spectrum computation. These two types of autocorrection functions are not rigorously equal, but within the harmonic oscillator model, if the quantum and classical oscillators both reach equilibrium at the temperature T within a canonical ensemble, the Fourier transforms of quantum and classical position autocorrelation functions only differ by a frequency-dependent prefactor,