These forces between two molecules can be expressed in a generalized relation

where is the total interaction energy (or potential) function obtained by summing up all pair-wise interactions between the molecules, and is the generalized force.

If is a negative quantity, it means that the force is attractive in nature, and vice versa. Therefore, having an insight into molecular level interactions paves the way to understand the macroscopic properties of such a system.

                         Figure 1.6.   An example of an energy function

Consider an example of an energy function (Fig. 1.6). Three regions can be observed here. In region 1, the slope of the potential function is positive which makes negative. Thus, this is the region where the forces are attractive in nature. The dispersion in such a state would not be stable because the particles would tend to come close to each other. Similarly, in region 2, is positive which makes the forces repulsive.