I know that particles in a solid are not fixed. They are constantly vibrating about their mean position. That is because of their kinetic energy. I was imagining a situation, lets say solid moves towards left of it's mean position. then the inter-molecular forces of attraction on the left side become slightly larger than that present on the right. But I also have to consider the repulsion between the electron clouds and the nucleus. But, obviously the forces acting are not in equilibrium, that is why the solid particles are moving around. So, what exactly is happening. how are the forces able to compensate? The vibration that is happening, are those SHM. Then the forces will be able to change depending upon the mean position.