Text: "The short-range forces are repulsive. This is usually the case for molecules which do not interact chemically. The steeply rising part of the curve near the origin is then due to the overlapping of electron clouds; that the forces correspond to it are called exchange forces (a term expressing the mathematical origin of these forces, which arise mathematically from certain exchange properties of the molecular energy state functions). Exchange forces can also be attractive. This is the case for atoms which enter into chemical combination with one another. Attractive exchange forces are called valence forces and play an important role throughout the field of chemistry. "Repulsive exchange forces account for the mechanical rigidity, or impenetrability, of molecules; the steepness of the potential-energy curve in the exchange-force domain gives rise to the limited compressibility of matter."

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