  The classical work by Zwanzig [J. Stat. Phys. 9 (1973) 215-220] derived
Langevin dynamics from a Hamiltonian system of a heavy particle coupled to a
heat bath. This work extends Zwanzig's model to a quantum system and formulates
a more general coupling between a particle system and a heat bath. The main
result proves that ab initio Langevin molecular dynamics, with a certain rank
one friction matrix determined by the coupling, approximates for any
temperature canonical quantum observables, based on the system coordinates,
more accurately than any Hamiltonian system in these coordinates, for large
mass ratio between the system and the heat bath nuclei.
