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usually the density operator for the microcanonical ensemble is given by

[tex]\rho = \sum_n p_n|n><n|[/tex]

where |n> are energy eigenstates and p_n is the probability that our system is in this state.

p_n = const. if the energy corresponding to |n> is in the energy inteval (E,E+∆E), otherwise p_n =0.

I.e. we assume, our system is composed of energy-eigenstates. Why is this allowed? Why don't we have to assume, that our system is composed of general quantum states? (with the same conditions for p_n, that is, p_n should be const. if the energy (or much better the expectation value for the energy, since we have no energy eigenstates anymore) corresponding to the general quantum state is within the energy interval)

--derivator

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# Microcanonical ensemble, density operator

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