The effect of counterpoise correction and relaxation energy term to the internal rotation barriers: Application to the BF₃⋯ NH₃ and C₂H₄ ⋯SO₂ dimers

The relevance of the fragment relaxation energy term and the effect of the basis set superposition error on the geometry of the BF₃⋯NH₃ and C₂H₄⋯SO₂ van der Waals dimers have been analyzed. Second-order Møller-Plesset perturbation theory calculations with the d95(d,p) basis set have been used to calculate the counterpoise-corrected barrier height for the internal rotations. These barriers have been obtained by relocating the stationary points on the counterpoise-corrected potential energy surface of the processes involved. The fragment relaxation energy can have a large influence on both the intermolecular parameters and barrier height. The counterpoise correction has proved to be important for these systems.