HYBRID METHODS OF MODELING COMPLEX MOLECULAR SYSTEMS

HYBRID METHODS OF MODELING COMPLEX MOLECULAR SYSTEMS 

After showing in the previous sections that the real place of the hybrid schemes in quantum chemistry is much more important than one may think, as almost every quantum chemistry method developed so far is hybrid explicitly or implicitly, we turn to a description of the existing hybrid methods understood in the narrow sense namely, those where a part of a system is described by a quantum chemistry method and other parts by MM methods described in previous sections. 

From the point of view of general theory described in Section 1.7.2 the relevant classification of QM/MM methods should be based on an assessment of the level to which the key elements of this theory are treated in that or any other specific hybrid scheme. The authors of [234] made a step in this direction and proposed a classification of hybrid schemes based on the interaction between the quantum and classical fragments. Such a classification of the QM/MM schemes is much more informative. It is built around the hypothetical representation of the total energy of the complex system comprising the quantitatively and classically treated subsystems in the form  

The origin of eq. (2.138) is well known the Hamiltonian for the complex system comprising two parts can be (and had been) written in analogous form  

With the above reservations we turn back to the interaction based classification of the hybrid QM/MM methods which allows us to distinguish the mechanical embedding, polarization embedding etc. We shall consider them subsequently. 

---