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

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.