The role of symmetric-stretch vibration in asymmetric-stretch vibrational frequency shift: the case of 2CH excitation infrared spectra of acetylene-hydrogen van der waals complex

Abstract

Direct infrared spectra predictions for van der Waals (vdW) complexes rely on accurate intra-molecular vibrationally excited inter-molecular potential. Due to computational cost increasing with number of freedom, constructing an effective reduced-dimension potential energy surface, which only includes direct relevant intra- molecular modes, is the most feasible way and widely used in the recent potential studies. However, because of strong intra-molecular vibrational coupling, some indirect relevant modes are also play important roles in simulating infrared spectra of vdW complexes. The questions are how many intra-molecular modes are needed, and which modes are most important in determining the effective potential and direct infrared spectra simulations. Here, we explore these issues using a simple, flexible and efficient vibration-averaged approach, and apply the method to vdW complex \chem{C_2H_2-H_2}. With initial examination of the intra-molecular vibrational coupling, an effective seven-dimensional {\it ab initio} potential energy surface(PES) for \chem{C_2H_2-H_2}, which explicitly takes into account the $Q_1$,$Q_2$ symmetric-stretch and $Q_3$ asymmetric-stretch normal modes of the \chem{C_2H_2} monomer, has been generated. Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for \nub{3}(\chem{C_2H_2})=0 and 1 to the Morse/long-range(MLR) potential function form. We provide the first prediction of the infrared spectra and band origin shifts for \chem{C_2H_2-H_2} dimer. We particularly examine the dependence of the symmetric-stretch normal mode on asymmetric-stretch frequency shift for the complex.