Generalized collective modes for the Lennard-Jones fluid

Abstract

Abstract The generalized collective mode approach proposed by de Schepper, Cohen and co-workers (Kamgar-Parsi, B., Cohen, E. G. D., and de Schepper, I. M., 1987, Phys. Rev. A, 35, 4781; de Schepper, I. M., Cohen, E. G. D., Bruin, C., van Rijs, J. C., Montfrooij, W., and de Graaf, L. A., 1988, Phys. Rev. A, 38, 271) for the investigation of time correlation functions of dense fluids is developed in a computer-adapted form which is free of any adjustable parameters. The generalized collective mode spectra of a Lennard-Jones fluid at reduced density n* = nσ3 LJ = 0·845 and reduced temperature T* = k B T/εLJ = 1·706 have been calculated using molecular dynamics for the seven-mode description of the longitudinal fluctuations and also for the three-mode description of the transverse fluctuations. The results are presented as functions of wavevector k for reduced wavenumbers 0 < kσLJ < 24. For longitudinal fluctuations two new kinetic modes are found in addition to the five modes known previously. It is shown that mode-coupling effects can be observed in this approximation, and the mode-coupling parameter of the sound dispersion is determined. For the transverse fluctuations we found the generalized hydrodynamic and two kinetic modes. The problem of the existence of a transverse propagating mode is discussed. A comparison of previous work with the results of lower-order approximations has been made.