Nakajima–Zwanzig equation

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The Nakajima–Zwanzig equation (named after the physicists Sadao Nakajima and Robert Zwanzig) 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 theory within the statistical mechanics of irreversible processes (named after Hazime Mori). By means of a projection operator the dynamics is split into a slow, collective part (relevant part) and a rapidly fluctuating irrelevant part. The goal is to develop dynamical equations for the collective part.


The starting point[1] is the quantum mechanical Liouville equation (von Neumann equation)

where the Liouville operator is defined as .

The density operator (density matrix) is split by means of a projection operator into two parts , where . The projection operator projects onto the aforementioned relevant part, for which an equation of motion is to be derived.

The Liouville – von Neumann equation can thus be represented as

The second line is formally solved as[2]

By plugging the solution into the first equation, we obtain the Nakajima–Zwanzig equation:

Under the assumption that the inhomogeneous term vanishes[3] and using

as well as

we obtain the final form


  • E. Fick, G. Sauermann: The Quantum Statistics of Dynamic Processes Springer-Verlag, 1983, ISBN 3-540-50824-4.
  • Heinz-Peter Breuer, Francesco Petruccione: Theory of Open Quantum Systems. Oxford, 2002 ISBN 9780198520634
  • Hermann Grabert Projection operator techniques in nonequilibrium statistical mechanics, Springer Tracts in Modern Physics, Band 95, 1982
  • R. Kühne, P. Reineker: Nakajima-Zwanzig's generalized master equation: Evaluation of the kernel of the integro-differential equation, Zeitschrift für Physik B (Condensed Matter), Band 31, 1978, S. 105–110, doi:10.1007/BF01320131

Original works[edit]

  • Sadao Nakajima (1958), "On Quantum Theory of Transport Phenomena" (in German), Progress of Theoretical Physics 20 (6): pp. 948–959 
  • Robert Zwanzig (1960), "Ensemble Method in the Theory of Irreversibility" (in German), Journal of Chemical Physics 33 (5): pp. 1338–1341 
  • original paper


  1. ^ A derivation analogous to that presented here is found, for instance, in Breuer, Petruccione The theory of open quantum systems, Oxford University Press 2002, S.443ff
  2. ^ To verify the equation it suffices to write the function under the integral as a derivative, deQLt'QeL(t-t') = -eQLt'QLPeL(t-t')dt' .
  3. ^ Such an assumption can be made if we assume that the irrelevant part of the density matrix is 0 at the initial time, so that the projector for t=0 is the identity.