An ab initio study of He–F2, Ne–F2, and Ar–F2 van der Waals complexes

Abstract

Single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)] has been used to calculate the ground state potential energy surfaces for He–F2, Ne–F2, and Ar–F2 van der Waals complexes. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions (aug-cc-pVTZ+bf). Single point calculations for approximate minima have also been performed with a larger quadruple zeta basis set (aug-cc-pVQZ+bf). For He–F2 and Ar–F2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne–F2 the CCSD(T) interaction energies of the two configurations are virtually the same. The linear configuration of each complex has been found to be much more sensitive than the T-shaped one to the changes of the F–F bond length with the interaction becoming weaker when the F–F bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. High-order correlation corrections have been found to play an important role in determining the relative stability of the linear and T-shaped configurations. The harmonic approximation zero-point vibrational energy for He–F2 exceeds the depth of both wells. For Ne–F2 the zero-point vibrational energy is greater for the linear configuration and, because of that, the complex has a T-shaped ground vibrational state. When the zero-point vibrational energy is taken into account for the Ar–F2 complex the linear and the T-shaped configurations are found to have nearly identical energies.