  At high pressure, the typical behavior of elements dictated by the periodic
table - including oxidation numbers, stoichiometries in compounds, and
reactivity, to name but a few - is altered dramatically. As pressure is
applied, the energetic ordering of atomic orbitals shifts, allowing core
orbitals to become chemically active, atypical electron configurations to
occur, and in some cases, non-atom-centered orbitals to form in the interstices
of solid structures. Strange stoichiometries, structures, and bonding motifs
result. Crystal structure prediction tools, not burdened by preconceived
notions about structural chemistry learned at atmospheric pressure, have been
applied to great success to explore phase diagrams at high pressure,
identifying novel structures in diverse chemical systems. Several of these have
been subsequently observed by experimental investigations, whose access to
high-pressure regimes is bolstered by advances in diamond anvil cell and
dynamic compression techniques. The joint efforts of experiment and theory have
led to particular success in the realm of high-temperature superconductors,
identifying many novel phases whose superconducting transition approaches room
temperature.
