### Abstract

For a two state system coupled to each other by a nonzero matrix element Δ and to the bath arbitrarily, the generalized master equation is derived by applying the well-known projection operator techniques to the quantum Liouville equation. The time-dependent rate kernel is expressed by an infinite summation of the perturbative terms in Fourier-Laplace space. The Schwinger's stationary variation principle in Hilbert space is extended to Liouville space and then applied to the resummation of the rate kernel. The Cini-Fubini-type trial state vector in Liouville space is used to calculate the variational parameters. It is found that the resulting stationary value for the rate kernel in Fourier-Laplace space is given by the [N,N-1]-Padé approximants, in the N-dimensional subspace constructed by the N perturbatively expanded Liouville space vectors. The (first-order) simplest approximation satisfying the variational principle turns out to be equal to the [1,0] Padé approximant instead of the second-order Fermi golden rule expression. Two well-known approximations, the noninteracting blip approximation (NIBA) and nonadiabatic approximation, are discussed in the context of the [1,0] Padé approximants, based on the variational principle. A higher-order approximation, [2,1] Padé approximant, is also briefly discussed.

Original language | English |
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Pages (from-to) | 2654-2661 |

Number of pages | 8 |

Journal | Journal of Chemical Physics |

Volume | 106 |

Issue number | 7 |

Publication status | Published - 1997 Feb 15 |

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### ASJC Scopus subject areas

- Atomic and Molecular Physics, and Optics

### Cite this

*Journal of Chemical Physics*,

*106*(7), 2654-2661.