Interaction-site-model description of collective excitations in classical molecular fluids

We describe a molecular theory for liquid dynamics that provides a method for calculating dynamical correlation functions of classical molecular fluids. The theory is based on the generalized Langevin equation and on the interaction-site model for molecular liquids. A simple model for memory functions is developed by generalizing the conventional one that has been successfully applied to monatomic systems. The theory is applied to the calculation of longitudinal current spectra of a model diatomic liquid and collective excitations in this solvent are investigated. We also clarify how these excitations originate from the translational and rotational motions of molecules.