Velocity Autocorrelation and Harmonic Motion in Supercooled Nondiffusing   Monatomic Liquids
  Studies of the many-body potential surface of liquid sodium have shown that it consists of a great many intersecting nearly harmonic valleys, a large fraction of which have the same frequency spectra. This suggests that a sufficiently supercooled state of this system, remaining in a single valley, would execute nearly harmonic motion. To test this hypothesis, we have compared $\hat{Z}(t)$, the normalized velocity autocorrelation function, calculated from MD simulations to that predicted under the assumption of purely harmonic motion. We find nearly perfect agreement between the two, suggesting that the harmonic approximation captures all essential features of the motion. 
Eric D. Chisolm, Brad E. Clements, and Duane C. Wallace