“…For either a pure state or a mixed state of an isolated system (i.e., an entire system), the time dependences of the density matrix can be derived from the time-dependent Schrödinger equation, which yields ,− i ℏ .1em ∂ ∂ t .1em bold-italicρ = [ H , ρ ] where H is the Hamiltonian matrix, and [,] denotes a commutator. This is similar in form to the classical Liouville theorem, , ∂ ∂ t .1em ρ = { H , ρ } where t is time, ρ is the density in phase space, H is the Hamiltonian, and {,} denotes a Poisson bracket.…”