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In equilibrium, the unifying framework of density-functional theory provides a variational scheme to determine both the thermodynamics and correlation functions of classical systems in the presence of an external potential field. Density-functional theory has found a range of application and nowadays forms a central pillar of liquid state and soft matter theory. Given the many successes it is natural to seek a variational approach to treat systems out of equilibrium. This was achieved with the development of the power functional theory. This review describes the approach of power functional theory, which is based on an exact one-body principle to describe the dynamics of overdamped, inertial classical, and quantum many-body systems.
Matthias Schmidt (Mon,) studied this question.