How to introduce thermodynamics to quantum mechanics? From numerous possibilities of solving this task, the simple choice is here: the conventional von Neumann equation deals with a density operator whose probability weights are time-independent. Because there is no reason apart from the reversible quantum mechanics that these weights have to be time-independent, this constraint is waived, which allows one to introduce thermodynamical concepts to quantum mechanics. This procedure is similar to that of Lindblad's equation, but different in principle. But beyond this simple starting point, the applied thermodynamical concepts of discrete systems may perform a 'source theory' for other versions of phenomenological quantum thermodynamics. This article is part of the theme issue 'Fundamental aspects of nonequilibrium thermodynamics'.
Keywords: compound systems; discrete systems; modified von Neumann equation; propagator; quantum thermodynamics.