Quasi-energy ansatz

The last equation is a variation principle for the coupled cluster quasi-energy and wavefunction within oscillating harmonic external fields. If one inserts a perturbation and Fourier expansion as ansatz for the cluster amplitudes... 

The self-energy of the right electrode is similarly defined. How to define the lesser (greater) self-energy, which represents the scatter-in (out) function ofelectrons provided by electrodes, is the central issue. Practical applications of the NEGF will be possible if the (generalized) Kadanoff-Baym ansatz is applicable [67-69, 77]. In the E-M-E system under constant bias Vb, electrodes are the electron reservoirs by means of the Landauer picture thus they can be approximated as a non-interacting quasi-equilibrium system,... 

This implies that when the ZORA ansatz is employed, the small components approach zero at the nuclei. The singular behaviour encountered in quasi-relativistic approaches based on kinetic-energy balance condition is therefore avoided. The ZORA method will be discussed in more detail in the next Section, and now we just note that the ZORA ansatz has a couple of desirable properties that shall be taken into consideration when the general ansatz function is constructed. 

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