Atom-diatom intermolecular forces and three body dispersion coefficients for doublet Li${}_3$

We calculate {\em ab initio} the ground state interaction potential surface for the lithium doublet trimer for both long range and near equilibrium geometries. A variety of methods are used to calculate the interaction energy, including complete active space SCF, full valence configuration interaction and coupled cluster theories as appropriate. Interpolation between {\em ab initio} points is accomplished with a dual-level interpolant moving least squares fitting algorithm using a scaled gaussian weight function. The global potential energy surface is found to have a large three-body interaction contribution. The atom-diatom dispersion coefficients ($C^ {\lambda}_n(r,\theta)$) for long range interactions ($R\gg r $) are found by fitting to the {\em ab initio} surface.