Generalized collective mode approach in the dielectric theory of dipolar systems

The concept of generalized collective modes, recently used for the investigation of simple fluids, is now applied to dipolar systems. The proposed approach is an extension of the three-variable theory of Madden and Kivelson [Adv. Chem. Phys. 56 (1984) 467] to arbitrary numbers of dynamical variables and values of wavelengths. Analytical expressions for the wave-vector- and frequency-dependent dielectric permittivity are obtained in terms of generalized dipolar modes. The collective mode spectra of longitudinal and transverse dipole moment fluctuations are evaluated for a Stockmayer fluid in a wide scale of wave numbers within up to the three-mode description. A comparison between molecular dynamics data and results of the generalized mode approach has showed that the dielectric permittivity of a dipolar fluid can be reproduced quantitatively in the whole region of wave vectors and frequencies using only several wavelength-dependent physically meaningful quantities. The Gaussian shape of time-dependent dipole moment fluctuations is examined in the infinite wave-vector limit