Linearization approximations and Liouville quantum-classical dynamics

We show that linearization methods, commonly used to approximate the evolution of the density operator in mixed quantum-classical systems, can be justified when a small parameter, the ratio of masses of the quantum subsystem and bath, is introduced. The same parameter enters in the derivation of the quantum-classical Liouville equation. Although its original derivation followed from a different formalism, here we show that the basis-free form of the quantum-classical Liouville equation for the density operator can also be obtained by linearization of the exact time evolution of this operator. These results show the equivalence among various quantum-classical schemes. (C) 2009 Elsevier B.V. All rights reserved