Models for bond distance dependent alkali dimer–rare gas potentials

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Abstract
Analytic expressions for Li2–Rg and Na2–Rg electronic ground state potentials (Rg=He,Ne,Ar,Kr,Xe) that depend on all three Jacobian coordinates, atom–molecule distance R, bond length r, and orientation angle γ, are presented. The potentials are obtained by expanding the repulsive and attractive parts in a two-term Legendre expansion and connecting them via generalized damping functions as proposed by Tang and Toennies [J. Chem. Phys. 80, 3726 (1984)]. The bond distance dependence (req=5.051 a.u.≤r≤6.838 a.u. for Li2 and req=5.809 a.u.≤r≤7.607 a.u. for Na2) is introduced in the repulsive part via the r-dependent shift of the repulsive barrier and in the attractive part via the r dependence of the multipole polarizabilities and effective excitation energies. These potentials are shown to agree reasonably with ab initio data. The resulting values of potential well depth ε and equilibrium distance Rm are used to check the validity of recently proposed direct combining rules for r-dependent potentials.