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The hierarchical equation of motion for a system in a harmonic Markovian bath is<ref name=Tanimura>{{Citation | last = Tanimura| first = Yoshitaka | year = 1990 | authorlink = Yoshitaka Tanimura |title=Nonperturbative expansion method for a quantum system coupled to a harmonic-oscillator bath | journal = Phys. Rev. A | volume = 41| issue = 12 |pages= 6676–6687 | doi = 10.1103/PhysRevA.41.6676 | pmid = 9903081 | bibcode = 1990PhRvA..41.6676T }}</ref>
:<math> \frac{\partial}{\partial t}{\hat{\rho}}_n = - \left(\frac{i}{\hbar}\hat{H}^{\times}_A + n\gamma\right) \hat{\rho}_n - {i\over\hbar}\hat{V}^{\times}\hat{\rho}_{n+1} + {in\over\hbar}\hat{\Theta}\hat{\rho}_{n-1}</math>
== Hierarchical equations of motion ==
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Using a two level spin-boson system Hamiltonian
:<math> \hat{H} = \hat{H}_A(\hat{a}^{+},\hat{a}^{-}) + V(\hat{a}^{+},\hat{a}^{-})\sum_{j}c_j\hat{x}_j + \sum_{j}\
Characterising the bath phonons by the spectral density <math> J(\omega) = \sum\nolimits_j c_j^{2}\delta(\omega - \omega_j)</math>
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By writing the density matrix in path integral notation and making use of Feynman–Vernon influence functional, all the bath coordinates in the interaction terms can be grouped into this influence functional which in some specific cases can be calculated in closed form. Assuming a high temperature heat bath with the Drude spectral distribution <math> J(\omega) = \hbar\eta\gamma^2\omega/\pi(\gamma^2 + \omega^2) </math> and taking the time derivative of the path integral form density matrix the equation and writing it in hierarchal form yields
:<math> \frac{\partial}{\partial t}{\hat{\rho}}_n = - \left(\frac{i}{\hbar}\hat{H}^{\times}_A + n\gamma \right) \hat{\rho}_n - {i\over\hbar}\hat{V}^{\times}\hat{\rho}_{n+1} + {in\over\hbar}\hat{\Theta}\hat{\rho}_{n-1}</math>
where <math> \Theta </math> destroys system excitation and hence can be referred to as the relaxation operator.
:<math> \hat{\Theta} = -\frac{\eta\gamma}{\beta} \
The second term in <math>\hat{\Theta} </math> is the temperature correction term with the inverse temperature <math> \beta = 1/k_B T</math> and the "Hyper-operator" notation is introduced.
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As with the Kubo's stochastic Liouville equation in hierarchal form, the counter <math> n </math> can go up to infinity which is a problem numerically, however Tanimura and Kubo provide a method by which the infinite hierarchy can be truncated to a finite set of <math> N </math> differential equations where <math> N </math> is determined by some constraint sensitive to the characteristics of the system i.e. frequency, amplitude of fluctuations, bath coupling etc. The "Terminator" defines the depth of the hierarchy. A simple relation to eliminate the <math> \hat{\rho}_{n+1}</math> term is found. <math>\ \hat{\rho}_{N+1} = - \hat{\Theta} \hat{\rho}_N/ \hbar\gamma</math>.<ref name=Tanimura91>{{Citation |last=Tanimura |first=Yoshitaka |authorlink= Yoshitaka Tanimura| author2= Wolynes, Peter | year = 1991 | title=Quantum and classical Fokker-Planck equations for a Gaussian-Markovian noise bath| journal = Phys. Rev. A | volume = 43 |issue=8 |pages=4131–4142 |doi=10.1103/PhysRevA.43.4131 |pmid=9905511 |bibcode=1991PhRvA..43.4131T }}</ref> With this terminator the hierarchy is closed at the depth <math> N </math> of the hierarchy by the final term:
:<math> \frac{\partial}{\partial t}{\hat{\rho}}_N = -\left( \frac{i}{\hbar}\hat{H}^{\times}_A + N\gamma \right) \hat{\rho}_N - {i\over \gamma\hbar^2}\hat{V}^{\times}\hat{\Theta}\hat{\rho}_{N} + {iN\over\hbar}\hat{\Theta}\hat{\rho}_{N-1}</math>.
The statistical nature of the HEOM approach allows information about the bath noise and system response to be encoded into the equation of motion doctoring the infinite energy problem of Kubo's SLE by introducing the relaxation operator ensuring a return to equilibrium.
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