The dynamic Jahn–Teller effect in the electronic ground state of Li3. An a b i n i t i o calculation of the BO hypersurface and the lowest vibronic states of Li3

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Abstract
The lowest adiabatic potential energy hypersurface of Li3 is determined using the coupled electron pair approximation (CEPA). The GTO basis used accounts for at least 80% of the valence electron correlation energy. Our calculated binding and ionization energies (34 kcal/mole and 4.0 eV, respectively) are in good agreement with the experimental values of Wu [J. Chem. Phys. 65, 3181 (1976)] (41.5±4 kcal/mole and 4.35±0.2 eV, respectively). We show that vibronic coupling is essential in the electronic ground state of Li3, giving rise to a dynamic Jahn–Teller effect which is treated in a nonharmonic approximation. The lowest vibronic states of Li3 have been calculated by solving the two‐dimensional Schrödinger equation for the E′×e′ Jahn–Teller case by a variational method. Vibrational probability densities are presented which allow the discussion of the ground state geometry of Li3. In contrast to previous work, an effective D3h geometry is proposed. This is supported by the calculated rotational energy levels, which can be interpreted as belonging to a symmetric top molecule with spectroscopic constants A=B=0.568 cm−1 and C=0.268 cm−1 (assuming three 7Li isotopes).