Instantaneous normal mode theory of more complicated correlation functions: Third- and fifth-order optical response

A general procedure is given for calculation of time correlations in liquid s, including complex multitime functions, with instantaneous normal modes ( INM). The method requires equilibrium averages only and eliminates any need to invoke a fictitious harmonic well. The third and fifth order optical re sponse functions, and the self- and collective-density correlations, are ob tained and compared with the results of other methods. INM provides several advantages for evaluating the nonlinear response function required for fif th-order scattering. The time expansions of the response functions, and the role of anharmonicity, are discussed. (C) 2000 American Institute of Physi cs. [S0021-9606(99)51646-2].