Ab initio calculation of rovibrational energy levels and dynamics of van der Waals complexes

A full three-dimensional potential energy surface (PES) for the Ar:HBr dimer has been calculated using 6–31 l++G(3df, 3 pd) basis set and second order Mø1ler-Plesset perturbation (MP2) theory. The global minimum is the H bonded structure with R e = 8.181 a.u., &thetas; = 0.0° and re = 2.673 a.u. and the van der Waals structure (vdW) with Re = 7.237 a.u. and &thetas; = 180° appears 6.6 cm−1 above the H bonded structure. The PES was fitted using the reproducing kernel of the Hilbert space (RKHS) multidimensional interpolation technique. Using an adiabatic separation of the HBr stretching coordinate from the intermolecular modes, the fitted PES was used to calculate the rovibrational energy levels of the complex in the v1 = 0, 1 vibrational states of the HBr using the vibrational configuration interaction (VCI) method. For the vdW structure the rovibrational energy levels, rotational constants and the red shift of the HBr frequency are in good agreement with experiment; however, at the H bond structure the agreement is less satisfactory.;A four-dimensional intermolecular interaction potential has been calculated for the (HBr)2 dimer at MP2 level using 6–311++G(3 df, 3pd) basis set. The potential is characterized by a minimum energy pathway where the global minimum occurs at the H bond structure with the center of mass bond distance RCM = 7.75 a.u. and angles &thetas;A = 10°, &thetas; B = 100° and well depth of 692.2 cm−1. The barrier for the H interchange occurs at the closed C2h structure for which RCM = 7.69 a.u., &thetas; A= 45°, &thetas;B = 135°and the barrier height is 74.1 cm−1. The PES was fitted using a combined least squares and RKHS interpolation method. The rovibrational energy levels of the complex were calculated by the split pseudo-spectral (SPS) method. The rovibrational energy levels and the rotational constants calculated are in good agreement with the experimental values. The dynamic effects of centrifugal distortion in the spectroscopic constants of the dimer were found to be very small. A low K, AK, the rotational constant of the dimer along the α-axis is K invariant, which is characteristic of a complex with an L-type configuration.