An attractive interaction reduces the energy of two nearby atoms.

For an attractive interaction between particles, the diffusion coefficient tends to decrease as concentration increases.

The phonons cause attractive interactions between some electrons, causing them to form Cooper pairs.

Imagine, for example, that we have a system of free (i.e. non-interacting) particles, to which an attractive interaction is introduced.

The next term is the attractive interaction of the electrons with the nuclei through the Coulomb potential .

Replies: The question really is, how to get an attractive interaction.

So, how can we get an attractive interaction?

The stretching of the chain gives rise to the above mentioned attractive interactions due to chain's rubber elasticity.

This tends to create strong, attractive interactions between water molecules.

In addition, water molecules can form "hydrogen bonds" to one another, which is an even stronger attractive interaction.