  Group I elements - alkali metals Li, Na, K, Rb and Cs - are examples of
simple metals with one s electron in the valence band. Under pressure these
elements display unusually complex structural behaviour transforming from
close-packed to low symmetry open structures. Unexpectedly complex form was
found for melting curves of alkalis under compression with initial increasing
in accordance to Lindemann criterion and further decreasing to very low melting
point. To understand complex and low symmetry structures in compressed alkalis
a transformation of the electron energy levels was suggested which involves an
overlap between the valence band and outer core electrons. Within the model of
the Fermi sphere - Brillouin zone interaction one can understand the complex
melting curve of alkalis.
