Molecular Mechanics

In the last section we have seen how hard is to obtain the energy of a molecular system. Many integral computations, many diagonalizations and transformations are needed to obtain, for example, the relative stability of a molecular conformation.

This is why, parallel to the methods of Quantum Chemistry there has always been the thinking that we could obtain similar results if we introduce all the qualitative chemical knowledge about molecular structure into a parametric function. That is, the strength of a chemical bond between two atoms, the steric hindrance, dispersion forces, hydrogen bonds, electrostatics ...All these interactions could be put together as a sum of analytical functions that give as a result a parametric energy function of the nuclear coordinates. This energy function is called an empirical force field and the strategy is the Molecular Mechanics (MM).

Historically there are several analytical functions that describe the inter and intramolecular interactions. Some examples of the oldest expressions are the Morse potential for the chemical bond, the Lennard-Jones or the Buckingham potential for the intermolecular interactions, the LEPS procedure ...and many others. However it is not until the end of sixties and the beginning of seventies with the work of Lifson, Allinger and Scheraga, and with the help of the emerging computers that some useful results are obtained. These Molecular Mechanics parametric functions need to be parameterized according to experimental results or ab initio calculations. We can find many different force fields in the literature. A force field will be characterized by the number and functional type of the energy terms and by the way the parameters are obtained.