THE SPIN-COUPLED DESCRIPTION OF LITHIUM CLUSTERS .2. PLANAR ARRANGEMENTS OF RHOMBUS UNITS

Spin-coupled theory is applied to the valence electrons of small planar lithium clusters constructed from rhombus units sharing vertices or edges. The study is motivated by the special importance of the rhombus unit in small lithium clusters and in fragments of the body-centred cubic (bcc) lattice cut according to the 110 plane. The spin-coupled descriptions which emerge for idealized fragments of the bcc 110 plane are based on orbitals which are localized between different nuclei, and which may be termed interstitial orbitals or IOs. Many of the IOs are associated with external edges of the clusters and exhibit only minor variations from cluster to cluster or between different spin multiplicities. A key feature in determining the locations of the IOs for a given cluster appears to be the difference between the number of valence electrons and the number of external edges. Even for those systems which have orbitals located in the interior, all of the orbitals retain interstitial rather than atom-centred forms. Certain characteristic features, which appear to be essentially independent of cluster size, may be important for the development of a spin-coupled theory of the electronic structure of metals.