A THEORETICAL-STUDY OF TUNNELING IN THE (HCCH)(2) COMPLEX

The internal motion in the acetylene dimer has been investigated at th e ab initio Moller-Plesset second-order perturbation theory (MP2) leve l, employing the double-zeta plus polarization function (DZP) basis se t. Basis set superposition errors (BSSE) corrections were included usi ng the counterpoise method. A two-dimensional (2D) Hamiltonian for the tunneling motion, considering the two bending modes in the dimer plan e was solved variationally, using as the potential energy function a t wo-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) tunne ling motion was obtained through a change of variables which allowed t he 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 tran sition frequencies are compared with the experimental data. The 1D Ham iltonian including anharmonicity corrections is shown to be a very eff icient and computational inexpensive procedure for treating the tunnel ing motion.