H igh pressure research  is a field of enormous relevance both for its sci-entific interest and for its industrial and technological applications. Many 3
materials undergo fascinating changes in their physical and chemical characteristics when subjected to extreme pressure. This behavior is caused by the involvement in bonding of electrons that would otherwise not be chemically active under zero-pressure conditions (the difference between zero pressure and atmospheric pressure is negligible from the point of view of its effect on materials properties, and we will use both concepts interchangeably). By and large, current chemical knowledge and the traditional rules for valence electrons are at a loss to explain most of the changes induced when materials are compressed, which makes chemical bonding under pressure an exciting research topic that has received much attention in recent years  (see Chapter 5).