A theoretical study of tunneling in the (HCCH)2 complex

Abstract

The internal motion in the acetylene dimer has been investigated at the ab initio Møller-Plesset second-order perturbation theory (MP2) level, employing the double-zeta plus polarization function (DZP) basis set. Basis set superposition errors (BSSE) corrections were included using the counterpoise method. A two-dimensional (2D) Hamiltonian for the tunneling motion, considering the two bending modes in the dimer plane was solved variationally, using as the potential energy function a two-dimensional ab initio intermolecular potential energy surface (PES). Coupling of the intramolecular vibration and dimer internal rotation has been neglected. Also, the synchronized one-dimensional (1D) tunneling motion was obtained through a change of variables which allowed the separation of the motion along the minimum energy path and the one perpendicular to it. Anharmonicity corrections were also added to the 1D procedure to reach the 2D results. The calculated splitting of transition frequencies are compared with the experimental data. The 1D Hamiltonian including anharmonicity corrections is shown to be a very efficient and computational inexpensive procedure for treating the tunneling motion.