Analytical Morse/long-range Potential Energy Surface And Predicted Ro-vibrational Spectra For H₂O-Rg?He, Ne And Ar? And H₂-C₂H₂ Van Der Waals Complexes With Larger-amplitude

Over the past 10 years, there have been major advances in experimental methods for spectroscopy, which led the requirements for more accurate calculations in theoretical studies. In addition, the microwave and infrared spectrum are used to study the ro-vibrational motions for molecular and clusters. Especially, the spectrum of van der Waals complexes with the larger-amplitude has always been a hot and difficult point. It is because that the vibration of inter-molecular is the anharmonic motion and couple with rotation. Thus, first and foremost, an accurate potential energy surface is required to allow for spectroscopic analysis for van der Waals complexes. In that case, some of the intramolecular modes may have low frequencies and large amplitudes as well, and will couple strongly to the intermolecular or van der Waals modes. Involving the intramolecular vibrational coordinates in the PESs and bound states calculations for van der Waal complexes is essential for fully and accurate predicting the vibrational spectra of the complexes.However, due to computational cost increasing with number of freedom,determining full dimensional intra- and inter-molecular potential energy surfaces are only limited to small size systems. In this paper, we study the H2O-Rg(He, Ne and Ar) and C2H2-H2 complexes to probe the origin and strength of intra- and inter-molecular vibrational couplings. Explicitly including or not including some specific intramolecular vibrational modes to study intermolecular interaction provides a precise theoretical way to examine the effects of anharmonic coupling between modes. Thus, 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. The microwave and infrared spectra have been predicted with these full- and reduced-dimension PESs and found to be in good agreement with the experimental values. Our mean results as follows:1. H2O-Ar system: A full-dimension intra- and inter-molecular ab initio potentialenergy surface(PES) for H2O–Ar, which explicitly incorporates interdependence on the intramolecular(Q1, Q2, Q3) normal-mode coordinates of the H2 O monomer, has been calculated. In addition, four analytic vibrational-quantum-state-specific PESs are obtained by least-squares fitting vibrationally averaged interaction energies for the(?1,?2, ?3) =(0, 0, 0),(0, 0, 1),(1, 0, 0),(0, 1, 0) states of H2 O to the three-dimensional Morse/long-range potential function. This showed that the resulting vibrationally averaged PESs provide good representations of the experimental infrared data, with root-mean-square(rms) discrepancies smaller than 0.06 cm-1for all rotational branches of the asymmetric stretch and the bend region fundamental transitions. Upon introduction of additional intramolecular degrees of freedom into the intermolecular potential energy surface, there is clear spectroscopic evidence of intra- and intermolecular vibrational couplings.2. H2O-He system: A new six-dimensional(6D) potential energy surface for H2O-He is presented which incorporates three intramolecular vibrational modes of H2 O, and is expected to be just as accurate as that for H2O-Ar. The intermolecular interaction of H2O-He is computed at the CCSD(T)/aug-cc-p VQZ level with bond functions. After vibrational averaging over three intramolecular coordinates, the resulting PESs were fitted to three-dimensional Morse/long-range(MLR) functional forms. Using these PESs, we first predicted the microwave and infrared spectra for para-H2O-He and ortho-H2O-He complexes, respectively.3. H2O-Ne system: To aid the spectral assignment, a four-dimension potential energy surface of H2O-Ne which depends on the intramolecular bending coordinate of the H2 O monomer and the three intermolecular vibrational coordinates has been constructed and the rovibrational transitions have been calculated. Three ortho and two para H2O-Ne bands have been identified from the experimental spectra. In our previous the calculated vibrational band shifts indicates that the coupling between the bending coordinate(Q2) of H2 O from the symmetric and anti-symmetric stretch coordinates(Q1 and Q3) is weak. Therefore a reduced-dimension treatment with only the bending coordinate(Q2) would be a reasonable approximation to calculate the mid-infrared transitions in the H2 O bending region.4. H2-C2H2 system: In this work, we extended to more complicated complex of H2-C2H2 with monomer beyond three atoms. With initial examination of the intra-molecular vibrational coupling, a six-dimensional ab initio potential energysurface(PES) for H2-C2H2, which explicitly takes into account the Q1symmetric-stretch and Q3 asymmetric-stretch normal-modes of the C2H2 monomer has been generated. We provide the first prediction of the infrared spectra and band origin shifts for C2H2-H2 dimer.