Multisite spin hopping analysis of multilevel dissipative quantum tunneling and coherence at finite temperatures

In the first article (I) of this series the real-time dynamics of a quantum mechanical (quasi-)particle in a double-well potential coupled to an ohmic environment at nonzero temperature has been studied ab initio on the basis of the Zwanzig-Caldeira-Leggett model Hamiltonian by means of a novel (pseudo-)spin analysis. The theory allows for a unified treatment of tunneling transport (“spin” flipping) and vibrational relaxation (“spin” hopping). The usual truncation to the ground state vibrational doublet is not made. In a second article (II) the dissipative (pseudo-)spin-12 dynamics has been evaluated explicitly in the weak coupling limit. The present article (III) implements appropriately adapted versions of the celebrated “displaced oscillators basis” and “noninteracting-blips approximation” into the general theory of part I in order to evaluate the “spin” dynamics in the strong coupling limit. Inter alia an investigation is presented of the so-called “duplex” (or double-doublet) system while the connection with the classical (thermal activation) regime is discussed in an appendix.