Generalized system-bath entanglement theorem for Gaussian environments

A system-bath entanglement theorem (SBET) with Gaussian environments was established previously in J. Chem. Phys. 152, 034102 (2020) in terms of linear response functions. This theorem connects the system-bath entanglement responses to the local system and bare bath ones. In this work, we generalize it to correlation functions. Key steps in derivation are the generalized Langevin dynamics for the hybridizing bath modes as in the previous work, together with the Bogoliubov transformation mapping the original finite-temperature canonical reservoir to an effective zero-temperature vacuum via an auxiliary bath. With the theorem, the system-bath entangled correlations and the bath modes correlations in the full composite space can be evaluated as long as the bare-bath statistical properties are known and the reduced system correlations are obtained. Numerical demonstrations are carried out for the evaluation of the solvation free energy of an electron transfer system with a certain intramolecular vibrational modes.