The dimer (HCl)2: Stationary points on the SCF 4-31G energy hypersurface and the role of entropy in the dimer formation

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Abstract
Two stationary points have been investigated on the 4-31G SCF energy hypersurface of (HCl)2. It has been shown, in agreement with experiment, that only the quasilinear structure satisfies all criteria required for a real minimum. Small heat of reaction and the crucial role of entropy in the (HCl)2 formation suggest that the dimer (HCl)2, in contrast to (HF)2, represents a true van der Waals molecule. A very small energy difference between the two stationary points (minimum and saddle point) suggests a very high rate of interconversion between two possible structures of the quasilinear form. Common features of and differences between (HCl)2 and (HF)2 are pointed out.