MgO, periclase, is the simplest oxide and has been a subject of intense experimental and theoretical study. Oxides and silicates make up the bulk of the Earth's mantle and crust, and thus it is important to understand and predict their behavior. The behavior of MgO, as the prototypical oxide, is the key to understanding mineral and rock behavior in the bulk of the Earth. An important feature of MgO is the nonrigid behavior of the oxygen O2– ion, which makes the interactions not describable by pairwise interactions. All other more complex oxides share this feature and add additional complications as well. MgO is simple in that it is an ionic material with no solid-state phase transitions until over 500 GPa according to the best computations. If we understand MgO we will not immediately understand all other oxides and silicates, but if we cannot understand MgO, we cannot understand any other oxide or silicate.

There is an enormous literature on MgO, and a comprehensive review is impossible here. Instead the main thrust is on the underlying fundamental physics of MgO. First-principles methods, as opposed to empirical methods, are emphasized. Properties that are addressed range from the thermal equation of state and elasticity to properties of defects, surfaces, and impurities.