On the Velocity Autocorrelation in a Classical Fluid

The evaluation of the velocity autocorrelation function and of its frequency spectrum in a classical liquid is discussed in the framework of the formalism developed by Tjon for the spin autocorrelation function. This is based on an integrodifferential equation involving a memory function, and is shown to be equivalent to the well-known approach based on the time expansion of the correlation function. The use of a Guassian memory function leads to fair agreement with the results of Rahman's numerical experiments for liquid argon.