Correlation energies in polyatomic molecules modelled in terms of bond order

Equilibrium geometries of low-lying isomers of some Li clusters, within Hartree–Fock theory plus bond order or MP2 correlation corrections

In a recent study by Kornath et al. [J. Chem. Phys. 118, 6957 (2003)], the Li(n) clusters with n=2, 4, and 8 have been isolated in argon matrices at 15 K and characterized by Raman spectroscopy. This has prompted us to carry out a theoretical study on such clusters up to n=10, using Hartree-Fock theory, plus low-order Møller-Plesset perturbation corrections. To check against the above study of Kornath et al., as a by-product we have made the same approximations for n=6 and 8 as we have for n=10. This has led us to emphasize trends with n through the Li(n) clusters for (i) ground-state energy, (ii) HOMO-LUMO energy gap, (iii) dissociation energy, and (iv) Hartree-Fock eigenvalue sum. The role of electron correlation in distinguishing between low-lying isomers is plainly crucial, and will need a combination of experiment and theory to obtain decisive results such as that of Kornath et al. for Li(8). In particular, it is shown that Hartree-Fock theory plus bond order correlations does account for the experimentally observed symmetry T(d) symmetry for Li(8).