The Quantum-Classical Density Operator for Electronically Excited Molecular Systems

Quantum Theory Project, University of Florida, Gainesville, FL 32611-8435, USA 

ADVANCES IN QUANTUM CHEMISTRY, VOLUME 47 ISSN 0065-3276 DOI I0.I0I6/S0065-3276(04)470I7-8 

This contribution deals with the description of molecular systems electronically excited by light or by collisions, in terms of the statistical density operator. The advantage of using the density operator instead of the more usual wavefunction is that with the former it is possible to develop a consistent treatment of a many-atom system in contact with a medium (or bath), and of its dissipative dynamics. A fully classical calculation is usually suitable for a many-atom system in its ground electronic state, but is not acceptable when the system gets electronically excited, so that a quantum treatment must then be introduced initially. The quantum mechanical density operator (DOp) satisfies the Liouville-von Neumann (L-vN) equation [1-3], which involves the Hamiltonian operator of the whole system. When the system of interest, or object, is only part of the whole, the treatment can be based on the reduced density operator (RDOp) of the object, which satisfies a modified L-vN equation including dissipative rates [4-7]. 

In what follows, we present a general coupled quantum-classical description using the PWT. We describe approximations suitable for short 

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The Quantum-Classical Density Operator for Electronically Excited Molecular Systems... 

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