What is Nakajima–Zwanzig equation?

The Nakajima–Zwanzig equation is an integral equation describing the time evolution of the "relevant" part of a quantum-mechanical system. It is formulated in the density matrix formalism and can be regarded a generalization of the master equation.

The equation belongs to the Mori-Zwanzig formalism within the statistical mechanics of irreversible processes. By means of a projection operator the dynamics is split into a slow, collective part and a rapidly fluctuating irrelevant part. The goal is to develop dynamical equations for the collective part.