The Pressing Role of Theory in Studies of Compressed Matter

Our chemical intuition, and indeed the periodic table, is based upon experience at 1 atm. However, in our universe, pressure spans an astounding 62 orders of magnitude (from interstellar space to the center of a neutron star). Chemistry is very different at high pressures, and because high-pressure experiments can be very difficult or even impossible to carry out, first-principles calculations are necessary to study matter at conditions of extreme pressure. In this chapter, we(i) describe computational techniques that can be employed to predict the structure of an extended system under pressure; (ii) describe how structure, chemical bonding, and electronic structure are affected by pressure; and (iii) provide examples of how theory has helped to interpret experimental data and provide predictions for experimental validation. We also comment on how well density functional theory does in predicting the structures and properties of compressed matter.