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 52, 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 Moller-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 Lin 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. (C) 2004 American Institute of Physics.