Lecture 11 : Non-covalent interactions 2 : Structures of Liquids
11.6
Potential Energy Functions
The form for the potential energy function is central to the success of any computer simulation. The forms are either empirical (determined by fitting to the experimental thermodynamic or kinetic data) or are obtained from the wavefunctions and energies of molecules. In this section we just mention two of the typical forms. If species i and j are spherical with diameters and charges qi and qj, the interaction energy may be written as
U ij (r ij ) = 4 [( /r ij )12 - ( /r ij )6] + qiqj/r ij
(11.13)
This is the well known Lennard-Jones (LJ) + Coulomb form.
The interaction between two molecules may be calculated if, for example, interaction sites can be placed at various atoms in the molecule. A model for water referred to as the simple charge model places a charge of -0.82e on oxygen and 0.42e on the hydrogens. The bond length between O and H is 1 and the bond angle is 109o 28'. This is shown in fig 11.5.
Figure 11.5 The Simple Point Change model of water.
and charges qi and qj, the interaction energy may be written as 
[( 
/r ij )6] + qiqj/r ij 
and the bond angle is 109o 28'. This is shown in fig 11.5.
