We investigated the mechanism of the third-order nonlinear optical (NLO) response of liquid CS2 by molecular dynamics simulation from both qualitative and quantitative aspects. The time-dependent relative importance of the orientational and collision-induced dynamics of the molecules to the fluctuation of polarizability was analyzed in order to understand the relation between the dynamics of the molecules and the third-order NLO response. Our analysis shows that the fluctuation of polarizability associated with the orientational dynamics of the molecules is responsible for 81% of the total NLO response. We calculated the values of the third-order NLO coefficients and they were found to be in semiquantitative agreement with the experimental values. We discuss the validity of fitting semiempirical theoretical models to the experimental signals as a means to analyze the mechanism of the NLO response. (C) 2000 American Institute of Physics. [S0021-9606(00)50114-0].