In addition to the NVT-NVE and direct NVT schemes, the direct NVE scheme has also been employed occasionally for vibrational spectrum calculations, although it was more used in an ad hoc manner with no theoretical justification yet. Here we will provide a brief theoretical foundation for the direct NVE scheme by revisiting the autocorrelation function of the harmonic model within a microcanonical ensemble. This is in contrast to the previous two schemes that are built on calculating the dipole derivative autocorrelation functions for a canonical ensemble equilibrated at a given temperature T .