Within the TFD model, the structural transitions observed in elemental metals under pressure could not have been foreseen. As an example let us consider cesium (Figure ), which adopts the bcc Cs-I phase at 1 atm. Under pressure the bcc phase first transforms to a face-centered-cubic (fcc) phase at 2.3 GPa, followed by a very complicated orthorhombic (space group C222 1 with 84 atoms per unit cell) phase at 4.2 GPa, and at ∼4.3 GPa Cs-III transforms to the non-close-packed tetragonal Cs-IV, which further undergoes a transition to the orthorhombic Cs-V 51 at ∼12 GPa, and finally to the double hexagonal close-packed Cs-VI at ∼70 GPa. In contrast to the conventional picture that increasing pressure yields closer-packed structures, the nearest neighbour coordination number first increases from 8 (bcc) to 12 (fcc), then decreases to about 10 (Cs-III), 8 (Cs-IV) and finally increases again to 10/11 (Cs-V) and 12 (Cs-VI). The electron density also does not behave according to the expectations from the TFD model; as shown in Figure (b) charge from around the atoms is transferred to regions along zigzag chains where no atoms are found.