NORMAL-MODE DYNAMICS IN SIMPLE LIQUIDS

A dynamical generalization of the instantaneous normal mode (INM) theo ry of liquid state dynamics is presented. Due to anharmonicities the e igenvalues and eigenvectors of the Hessian matrix change with time. Th erefore, regular INM theory gives a description of molecular dynamics valid only for short times. Starting out from the classical equations of motion the velocity correlation function is expressed in terms of a series of propagation matrices. These are calculated by diagonalizing the Hessian matrix at configurations equidistant in time along a shor t piece of trajectory. Correlation functions calculated by this normal mode propagation (NMP) for a representative selection of atomic syste ms agree quantitatively with results, from molecular-dynamics simulati on. (C) 1998 American Institute of Physics.