Bound and quasi-bound states of the Li center dot center dot center dot FHvan der Waals molecule

The fully dimensional potential energy surface of the ground electronic sta te of the Li . . . FH van der Waals complex was constructed by fitting ab i nitio energies obtained on a grid of ca. 2000 nuclear geometries. The ab in itio calculations were performed using the coupled-cluster approach with si ngle, double, and noniterative perturbative triple excitations [the CCSD(T) method]. The large and carefully optimized basis set, consisting of 140 or bitals, was employed. All CCSD(T) energies were corrected for the effects o f the basis set superposition error and deformation of the HF monomer in th e Li . . . FH complex. The basis set superposition error-corrected CCSD(T) potential energy surface is characterized by a relatively deep, 1991 cm(-1) , van der Waals well and a late barrier for the Li + HF --> LiF + H reactio n located at 2017 cm(-1) above the Li + HF asymptote. The Li . . . FH compl ex is bent (the Li-F-H angle is 109 degrees). The bending Li-F-H angle char acterizing the saddle point is 71 degrees. The fitted potential energy surf ace was used to calculate the bound and low-lying quasi-bound vibrational s tates of the Li . . . FH complex. The required re-vibrational calculations were performed within the framework of the Sutcliffe-Tennyson Hamiltonian f or triatomic molecules. The energy positions and widths of the quasi-bound states were obtained using the stabilization method. The re-vibrational pro blem was solved both variationally, by diagonalizing the Hamiltonian matrix in a discrete basis set, and by using the perturbative approach based on t he adiabatic separation of vibrational motions. All spectroscopic informati on obtained in this study was rationalized in terms of effective potentials for the van der Waals stretch and bend motions arising from the adiabatic separation of the high- and low-frequency modes. (C) 2000 John Wiley & Sons , Inc.