Effective group potentials: a powerful tool for hybrid QM/MM methods?

Abstract

Classical force fields are still extensively used for theoretical studies of solvated very large molecules. Since there is an extensive rearrangement of the electrons, which is neglected in the classical model, force field methods are inherently not able to describe bond breaking/forming in chemical reactions. Chemical reactions paths involve a small set of atoms. The rest of the molecule is spectator during the reaction however it is an important part, and taking into account its influence on the active part is mandatory. Hybrid methods integrate these concepts, and thus have been designed for calculating the active moiety by electronic structure methods (semi-empirical, ab initio or DFT…), while the rest is calculated by a force field method. The main problem with hybrid scheme is deciding how the two parts should be connected. In many cases the MM and QM parts belong to the same molecule, and the connection between the two parts must be made by cutting a molecular bond. After an overview of the effective group potential (EGP) theoretical background, successes of the method are reviewed, such as the validity of the pseudo-Cp in the case of the activation barrier of the following reaction: Cp 2LnH †+H 2→Cp 2LnH+HH † (Ln, the elements of the lanthanide series). The fourth part of the article will present how and why the EGP formalism in its actual development can contribute to the treatment of the QM/MM connectivity problem. Indeed, the EGPs allow to keep the chemical sense of the QM/MM border.