Thermal expansion

The force between atoms separated a distance R is zero. The curve is not linear about R. Heating the crystal increases the vibration amplitude, shifting the equilibrium position to larger values, and the crystal expands.

If the graph is drawn with PE on the vertical axis the equilibrium distance is at the bottom of the potential well, where the gradient is zero. The same argument applies. Increasing the temperature will shift the equilibrium position inward or outward, depending on the exact shape of the well.

This graph is often the focus of an exam question.

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