title: An intramolecular vibrationally excited intermolecular potential for He-OCS: Globally tested by simulation of vibrational shifts for OCS in HeNN = 1 - 100  Clusters.
abstract: An effective four-dimensional ab initio potential energy surface (PES) for He-OCS, which explicitly incorporates dependence on the intramolecular Q(1) (O-C) stretch normal mode of OCS and is parametrically dependent on its Q(3) (C-S) stretch coordinate has been calculated at the coupled-cluster single double triple/aug-cc-pVQZ level including bond functions. Analytic two-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for v(1)(OCS)= 0, and 1 to the Morse/long-range potential function form. These fits to 305 points both have root-mean-square (rms) deviation of 0.022 cm(-1), and require only 49 parameters. The resulting vibrationally averaged PESs provide good representations of the experimental microwave and infrared data: for 10 microwave transitions, and 51 infrared transitions of the He-OCS dimer, the rms discrepancies are only 93.2 MHz and 0.003 cm(-1), respectively, which are more than four times better than previous theoretical predictions on their original ab initio potentials. The calculated infrared band origin shift associated with the v(1) fundamental of OCS is blueshifted by 0.082 cm(-1) for He-OCS dimer, which is in good agreement with the experimental value of 0.111 cm(-1). The path integral Monte Carlo algorithm and a first order perturbation theory estimate are used to simulate the ν(1) vibrational band origin frequency shifts of OCS in He(n) clusters for  N = 1 - 100 . The predicted vibrational frequency shifts with first a blueshift for small N and then followed by a transition to a redshift for larger N are in excellent agreement with experiment across the whole range of N. These results for increasing N demonstrate the high quality of these potentials and globally test the accuracy not only the near global minimum, but also in regions not accessed by N = 1 He-OCS dimer.
authors.affiliation: Institute of Theoretical Chemistry, State Key Laboratory of Theoretical and Computational Chemistry, Jilin University, 2519 Jiefang Road, Changchun 130023, People's Republic of China. Prof_huili@jlu.edu.cn
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.forename: Yong-Tao
authors.initials: YT
authors.lastname: Ma