Nakajima–Zwanzig equation

The Nakajima–Zwanzig equation is a fundamental equation in the field of nonequilibrium statistical mechanics. It describes the time evolution of the reduced density matrix of a subsystem that is coupled to a larger environment. The equation provides a way to study the dynamics of a system when we are only interested in a part of it, often referred to as the "system" while the rest is treated as the "environment" or "bath.