Intersubsystem junction

The technical problem was to develop an adequate form of the intersubsystem junction for the case when the quantum subsystem reduces to the d-shell. The problem here is to keep clear advantage of the TFT taking into account the ligands electronic structure over the CFT reproducing it economically in the otherwise MM calculation. 

The technical problem is of course to develop an adequate form of the intersubsystem junction for the case when the quantum system is represented by the d-shell. This is done using the EHCF(L) technique described above. In the EHCF(L), the effective crystal field in agreement with the general theory of Section 1.7.2 is given in terms of the /-system Green s function. The natural way to go further with this technique is to apply the perturbation theory to obtain estimates of the /-system Green s function entering eqs. (4.83) and/or (4.92). That is what we shall do now. 

Correct form of intersubsystem junction corresponds to sequential separation of variables. This principle simply states that the construction should be made not in the ad hoc manner. 

In the case of intersubsystem junction represented by classical bonding terms an important question arises which terms should be included and which should not The most popular way is to include classical bonded force fields when at least one MM atom is involved in it [33,34]. At the same time it does not allow to avoid double counting of interactions computed quantum mechanically. To smooth this inconsistence the authors of Ref. [124] proposed to calculate only those classical bonding force fields where at least one central atom is from the MM subsystem or in the case of improper dihedral terms only those with both outer atoms from the MM subsystem. 

In Chapter 3 we introduce the formal construction and testing of an intermediate procedure bridging QM and MM procedures. This will be a mechanistic treatment, derived from the quantum description of the molecular system. Then this technique will be used to define the one-electron states of the frontier atoms - the key elements of the intersubsystem border/junction the shapes of the one-electron states at the frontier atoms, their electronic densities and the response of either subsystem to the variables characterizing each subsystem. 

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