Greens functions of molecular subsystems in the condensed phase

Markov approximation for Green s functions of molecular subsystems in the condensed phase 

Practically, any experimental study of an arbitrary system reduces to measuring the response of some physical characteristic A of the system to a probing external action which perturbs, in the general case, another characteristic B. The required response is then described by a conveniently calculable retarded Green s function (GF) that contains sufficiently complete information about the states of the system 144 

On the other hand, there exist well-developed methods for calculating states of subsystems using the Markov approximation for the reduced density matrix 

We find the linear response of a subsystem in contact with a reservoir to an external perturbation involving some variable B of the subsystem and depending on the time through a function F(t), so that the corresponding perturbation operator can be written in the form 

Here pq is the diagonal matrix element of the equilibrium statistical operator of 

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