Longitudinal modes, transverse modes and velocity correlations in liquids: II

For pt.I see ibid., vol.11, p.3749 (1978). A representation of the velocity autocorrelation function in terms of the longitudinal and transverse momentum current densities is applied to models of liquid argon and liquid sodium. Results for the contributions of the current components to both correlation functions and frequency spectra are presented. The existence of collective density excitations for wavelengths down to one or two atomic spacings is shown to be correlated with the oscillatory behaviour of the velocity correlation function. It is also argued that the formation of a peak in the frequency spectrum, at a frequency well below the Einstein frequency is indirect evidence that the liquid can sustain the propagation of shear wave modes.