Computational studies of the He-LiH system

We have computed an ab initio potential energy surface for the He-LiH system.We compute the He-LiH interaction energy at the CCSD(T) level using large correlation consistent atomic basis sets supplemented with bond functions. To capture the severe anisotropy of the He-LiH potential, we interpolate our ab initio points in the angular direction with cubic splines, then expand the splines in terms of Legendre Polynomials. We have constructed both a He-LiH rigid rotor potential and a complete He-LiH potential where the LiH bond length is allowed to change. The resulting potential surface has a unique shape. The He-LiH rigid rotor collinear geometry has a very attractive minimum of -176.7^-1 cm while the LiH-He collinear geometry hasa local minimum of only -9.8 cm^-1 Using our computed He-LiH potential energy surface, we investigate the collision dynamics of He-LiH motivated by the hypothesis that these collisions may have been important in the energy balance of the very early universe. Using a totally quantum mechanical treatment of the collision dynamics, we compute both pure rotational and rovibrational state-to-state cross sections. We integrate our rovibrational crosssections over a Maxwell-Boltzmann distribution of energies to obtain temperature dependent vibrational excitation and relaxation rate constants. The vibrational excitation rate constants are very small for temperatures below 400 K, but becomes significant at higher temperatures. These results suggests that He-LiH collisions probably were important in the early universe, especially in the larger primordial gas clouds.We also investigate the structure and dynamics of small Hcn-LIH clusters using diffusion quantum Monte Carlo techniques. We find that three-body effects are negligible, so we take the Hcn-LIH potential to be a pairwise additive potential; we use111the HFD-B3-FCI1 He-He potential of Aziz and Janzen [R. A. Aziz and A. R. Janzen,Phys. Rev. Lett. 74, 1586 (1995)] and our He-LiH potential. Because of the strongHe-LiH attraction, one helium is always located in the attractive well at the lithium end of LiH. This one helium atom appears to fill the first solvation shell. Additionalhelium atoms cluster around this "special" helium and make up a very diffuse second shell.