The electrostatic energy charges and dipoles

The interaction between point charges is given by the Coulomb potential, with e 

The MM2 and MM3 force fields use a bond dipole description for ei- The interaction between two dipoles is given by eq. (2.21). 

The fitting is done by minimizing an error function of the form shown in eq. (2.23), under the constraint that the sum of the partial charges (2, is equal to the total molecular charge. The electrostatic potential is sampled at a few thousand points in the near vicinity of the molecule. 

Obtaining a good description of the electrostatic interaction between molecules (or between different parts of the same molecule) is one of the big problems in force field work. Many commercial applications of force field methods are aimed at designing molecules that interact in a specific fashion. Such interactions are usually pure non-bonded, and for polar molecules such as proteins, the electrostatic interaction is very important. 

The modelling of the electrostatic energy by (fixed) atomic charges has four main deficiencies  

---