The potential energy curve for I2(X1Σ g+) is calculated by direct numerical integration of one of Klein's expressions, without making assumptions about the variation of spectroscopic constants with vibrational quantum number. At internuclear distances of greater than ∼6.5 A° the binding energy is found to be greater than that indicated by a Morse curve. This additional binding energy is attributed to van der Waals' forces. A value of the van der Waals' constant c = 3.30 × 106 cm-1 A°6 is obtained which agrees with theoretical predictions. © The Faraday Society and Contributors, 1964.
