Description of isolated macroscopic systems inside quantum mechanics

For an isolated macrosystem classical state parameters zeta (t) are introduced inside a quantum mechanical treatment. By a suitable mathematical representation of the actual preparation procedure in the time interval [T,t(0)] a statistical operator is constructed as a solution of the Liouville-von Neumann equation, exhibiting at time t the state parameters zeta (t'), t(0) less than or equal to t' less than or equal to t, and preparation parameters related to times T less than or equal to t' less than or equal to t(0). Relation with Zubarev's nonequilibrium statistical operator is discussed. A mechanism for memory loss is investigated and time evolution by a semigroup is obtained for a restricted set of relevant observables, slowly varying on a suitable time scale.