Validation of computations by high pressure measurements: Solutions to problems that experiments cannot solve

Calculations of the properties of condensed matter systems can be crucially tested by comparison with their experimental pressure behavior. When valid ated by such a comparison theoretical models become reliable computer exper iments and can be used to explore properties of the system which are intrin sically unobservable by experiment alone. Three examples are given of such methods: (i) The behavior of wurtzite indium nitride under pressure which is shown t o be due to a second-order isostructural phase transition. (ii) The instability of tetrahedrally bonded semiconductors (Ge) under pres sure which turns out to be related to a second-order process. (iii) The explanation of the lattice dynamics of alpha -boron at ambient co nditions which can be understood only by comparison of its high pressure be havior with calculations.