Local Harmonic Approximation to Quantum Mean Force Gibbs State

When the strength of interaction between a quantum system and bath is non-negligible, the equilibrium state can deviate from the Gibbs state. But the expression of such a mean force Gibbs state in an arbitrary parameter regime is unknown and is numerically challenging to determine. In this work, we first review the local harmonic approximation to this problem [Maier et al., Phys. Rev. E 81, 021107 (2010)], which can accurately determine the mean force Gibbs state when either the system-bath coupling or the temperature is large, or when the third and higher derivatives of the potential are small compared to certain system-bath specific parameters. In the appropriate limit, we show that the local harmonic approximation reduces to the ultra-strong coupling and high temperature results recently derived in the literature. After deriving an estimate for the error induced by this method, we apply it to study some systems, like a quartic oscillator and a particle in a quartic double-well potential. We also apply this method to analyze the proton tunneling problem in a DNA recently studied in literature [Slocombe et al., Comm. Phys., vol. 5, no. 1, p. 109, 2022], where our results suggest the equilibrium value of the probability of mutation to be orders of magnitude lower than the steady state value obtained there ($10^{-8}$ vs $10^{-4}$).