ABSTRACT
In the previous chapter we studied the Kubo formalism, Langevin equa-
tion, problems associated with the Langevin equation, and the Generalized
Langevin Equation (GLE). From the problems of the Langevin equation we
observed that the Drude formula is bound to be invalid in a general setting.
GLE leads to Generalized Drude Formula (GDF) in which the Drude scatter-
ing rate (i.e., the friction coefficient) acquires a frequency dependence and has
general validity. This frequency dependence is due to memory effects in the
friction coefficient and when these memory effects are neglected we get back
the simple Drude formula from GDF.
