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Chemical Dynamics in Condensed PhasesRelaxation, Transfer and Reactions in Condensed Molecular Systems$
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Abraham Nitzan

Print publication date: 2006

Print ISBN-13: 9780198529798

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780198529798.001.0001

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The Quantum Mechanical Density Operator And Its Time Evolution: Quantum Dynamics Using The Quantum Liouville Equation

The Quantum Mechanical Density Operator And Its Time Evolution: Quantum Dynamics Using The Quantum Liouville Equation

Chapter:
(p.347) 10 The Quantum Mechanical Density Operator And Its Time Evolution: Quantum Dynamics Using The Quantum Liouville Equation
Source:
Chemical Dynamics in Condensed Phases
Author(s):

Abraham Nitzan

Publisher:
Oxford University Press
DOI:10.1093/oso/9780198529798.003.0016

The starting point of the classical description of motion is the Newton equations that yield a phase space trajectory (rN (t), pN (t)) for a given initial condition (rN (0), pN (0)). Alternatively one may describe classical motion in the framework of the Liouville equation (Section (1.2.2)) that describes the time evolution of the phase space probability density f (rN , pN ; t). For a closed system fully described in terms of a well specified initial condition, the two descriptions are completely equivalent. Probabilistic treatment becomes essential in reduced descriptions that focus on parts of an overall system, as was demonstrated in Sections 5.1–5.3 for equilibrium systems, and in Chapters 7 and 8 that focus on the time evolution of classical systems that interact with their thermal environments. This chapter deals with the analogous quantum mechanical problem. Within the limitations imposed by its nature as expressed, for example, by Heisenbergtype uncertainty principles, the Schrödinger equation is deterministic. Obviously it describes a deterministic evolution of the quantum mechanical wavefunction. The analog of the phase space probability density f (rN , pN ; t) is now the quantum mechanical density operator (often referred to as the “density matrix”), whose time evolution is determined by the quantum Liouville equation. Again, when the system is fully described in terms of a well specified initial wavefunction, the two descriptions are equivalent. The density operator formalism can, however, be carried over to situations where the initial state of the system is not well characterized and/or a reduced description of part of the overall system is desired. Such situations are considered later in this chapter.

Keywords:   Bloch equations, Liouville equation, Markovian limit, Nakajima–Zwanzig equation, Redfield equation, Schrödinger representation, Wigner transform, coherences, density matrix

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