Time Correlations and Their Descriptions of Materials Properties

Time correlation functions of dynamical variables have been used ubiquitously to characterize various materials properties. In this chapter, we begin by discussing some general properties of time correlation functions, followed by characterizing thermal conductivity and viscosity using heat current and stress correlations, respectively, as some examples. We then move onto velocity autocorrelation functions that characterize atomic degrees of freedom, similar to normal modes presented in Chap.  2 .