Ab initio effective valence shell Hamiltonian calculations of Li2 potential curves

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Abstract
Potential curves are calculated for the lithium molecule using an effective valence shell Hamiltonian formulation of quasidegenerate many‐body perturbation theory and two different sets of molecular orbitals. One set utilizes the low lying valence molecular orbitals which are optimized for particular valence states, while the other set is simply taken as the union of the atomic valence sets. The shapes of the potential curves for the lowest four states (1Σ+g,3Σ+u,3Πu, and 1Σ+u) are in excellent agreement with those obtained by Olson and Konowalow from extensive MCSCF calculations and hence with experiments. The total energies are high by about 0.1 a.u. because our basis set does not describe the core electrons as well. The perturbation expansion is found to display excellent convergence properties between the second and third order calculations. A future paper will compare the bond length dependence of the effective matrix elements with those customarily assumed in semiempirical theories of bonding.

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