Corrections to the RPA Dielectric Function Induced by Fluctuations in the Momentum Distribution

In this article, we introduce a new dielectric function that is different even from the generalised RPA, first introduced in cond-mat/9810043, in that it pays attention to fluctuations in the momentum distribution. We argued that this is important when the momentum distribution is very nonideal. This new dielectric function reduces to the generalised RPA when the number-fluctuations are small, and the generalised RPA in turn reduces to the traditional RPA dielectric function when the momentum distribution is close to ideal.