Theoretical investigation of potential energy surface and bound states for the N₂-OCS van der Waals complex

In this work we report an ab initio intermolecular potential energy surface and theoretically spectroscopic studies for N₂-OCS complex. A four-dimensional intermolecular potential energy surface (4D PES) is constructed at the level of single and double excitation coupled-cluster method with a non-iterative perturbation treatment of triple excitations [CCSD(T)] with aug-cc-pVTZ basis set supplemented with bond functions. A global minimum corresponding to a planar and nearly T-shaped structure, which has been observed experimentally, is located at R = 3.96 Å, θ₁ = 8 or 172°, θ₂ = 75°, φ = 180 or 0° with a well depth of 271.078 cm^-1 on the potential energy surface. The local minimum corresponding to a linear geometry is also found at R = 5.06 Å, θ₁ = 2 or 178°, θ₂ = 179°, φ = 0 or 180° with a well depth of 224.743 cm^-1. The bound state calculations have been performed for the complex by approximating the N₂ and OCS molecules as the rigid rotors. The calculated structural parameters and rotational transition frequencies are in good agreement with the experimental observed values. Based on the ab initio PES, the tunneling splitting is calculated to be 0.052 cm^-1 for the ground vibrational state, which can just reproduce 64% of experimental observation (0.082 cm^-1). A refined method is used to calculate the tunneling splitting, and a much better result is obtained with the value of 0.094 cm^-1.