  The physicochemical behavior of elements and compounds is heavily altered by
high pressure. The occurrence of pressure-induced reactions and phase
transitions can be revealed by crystal structure prediction approaches. In this
work, we explore the C-H-O phase diagram up to 400 GPa exploiting an
evolutionary algorithm for crystal structure predictions along with ab initio
calculations. Besides uncovering new stable polymorphs of high-pressure
elements and known molecules, we predicted the formation of new compounds. A
2CH4:3H2 inclusion compound forms at low pressure and remains stable up to 215
GPa. Carbonic acid (H2CO3), highly unstable at ambient conditions, was
predicted to form exothermically at mild pressure (about 1 GPa). As pressure
rises, it polymerizes and, above 300 GPa, reacts with water to form
orthocarbonic acid (H4CO4). This unexpected high-pressure chemistry is
rationalized by analyzing charge density and electron localization function
distributions, and implications for general chemistry and planetary science are
also discussed.
