Applications of Diatomics-in-Molecules Theory. III. The Li4 System

Some simple diatomics-in-molecules relations for homonuclear four-atom systems with s-electron bonding are presented and applied to Li4 clusters of D4h, Td, and D2d symmetry. Predictions include a square Li4 molecule (with side 5.47 Bohr units) and an atomization energy 63.94 kcal/mole at 0°K and a disphenoidal (Li2)2 cluster (two diatomic Li2 species with perpendicular axes separated by 5.89 units) with atomization energy 60.52 kcal/mole at 0°K, both species stable with respect to isolated diatomics. The optimum energy of tetrahedral Li4 (side 5.8 Bohr units) lies about 11 kcal/mole above that of the square; however the tetrahedron exhibits Jahn–Teller instability with respect to the square and disphenoid conformations. Fundamental vibrational frequencies are calculated for both D4h and D2d species.