Local coordination in relation to electronic structure of condensed conducting phases

Density functional theory (DFT) can be employed to calculate an effective, density-dependent pair potential, mediated by the conduction electrons, in sp liquid metals. The total electronic charge around the metallic ion, say Na+, in an electron gas must perfectly screen the ion, as long-range electric fields cannot exist in a conducting medium. The pair potential thus obtained for liquid Na near freezing, which is consistent with high coordination number, is compared with that obtained by inversion of the liquid structure factor as measured in diffraction experiments. Results of such experiments on the heavy highly expanded liquid alkali Cs, which reveal low coordination numbers, have motivated recent DFT calculations on ordered chains of K atoms, both linear and zig-zag. The ground-state electronic properties are compared and contrasted with those of structures with higher coordination numbers such as diamond and body-centred cubic. Chains of C atoms in C2BN are then briefly referred to, and differences in conduction properties from recent DFT calculations on a hexagonal layer of CBN are stressed. Finally, some attention is given to the coexistence of metallic and molecular phases, with particular reference to the insulator-metal transition in crystalline I-2 under pressure and to very recent studies of fluid molecular H under extreme conditions of pressure and temperature.