Hybrid Force Field Electronic Structure Methods

Hybrid Force Field-Electronic Structure Methods... 

HYBRID FORCE FIELD ELECTRONIC STRUCTURE METHODS... 

The foregoing discussion of valence is. of course, a simplified one. From ihe development of the quantum theory and its application to the structure of the atom, there has ensued a quantum theory of valence and of the structure of the molecule, discussed in this hook under Molecule. Topics thal are basically important to modem views of molecular structure include, in addition to those already indicated the Schroedinger wave equation the molecular orbital method (introduced in the article on Molecule) as well as directed valence bonds bond energies, hybrid orbitals, the effect of Van der Waals forces and electron-dcticiem molecules. Some of these subjects are clearly beyond the space available in this book and its scope of treatment. Even more so is their use in interpretation of molecular structure. [However, sec Crystal Field Theory and Ligand.)... 

Hybrid potentials of the type discussed in this chapter have been in use for twenty five years or so. Some of the earliest examples were those designed to study conjugated organic molecules in which the -electrons of the system were treated with a semiempirical QM method and the cr-bonding framework was described with a MM force field [8, 9], The methods were used to study the structure and spectra of the molecules [10, 11] and photoisomerization processes [12, 13], The first true combined potential in which both a and w electrons were considered in the quantum mechanical region was also developed by Warshel, in collaboration with Levitt, which they used to study the mechanism of the enzyme, lysozyme [14], They combined a semiempirical QM method to describe a portion of the enzyme and the substrate, and a standard MM force field to describe the rest of the atoms. 

In recent years, there have been many attempts to combine the best of both worlds. Continuum solvent models (reaction field and variations thereof) are very popular now in quantum chemistry but they do not solve all problems, since the environment is treated in a static mean-field approximation. The Car-Parrinello method has found its way into chemistry and it is probably the most rigorous of the methods presently feasible. However, its computational cost allows only the study of systems of a few dozen atoms for periods of a few dozen picoseconds. Semiempirical cluster calculations on chromophores in solvent structures obtained from classical Monte Carlo calculations are discussed in the contribution of Coutinho and Canuto in this volume. In the present article, we describe our attempts with so-called hybrid or quantum-mechanical/molecular-mechanical (QM/MM) methods. These concentrate on the part of the system which is of primary interest (the reactants or the electronically excited solute, say) and treat it by semiempirical quantum chemistry. The rest of the system (solvent, surface, outer part of enzyme) is described by a classical force field. With this, we hope to incorporate the essential influence of the in itself uninteresting environment on the dynamics of the primary system. The approach lacks the rigour of the Car-Parrinello scheme but it allows us to surround a primary system of up to a few dozen atoms by an environment of several ten thousand atoms and run the whole system for several hundred thousand time steps which is equivalent to several hundred picoseconds. 

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