When matter experiences pressure higher than the typical atomic pressure, then the atomic structure will break down and collapse to a more rigid form. The typical pressure is just the binding energy divided by the volume of the atom. For hydrogen, this comes out to be about 15 TPa. The same principle can be used for molecules, only then the typical energies are smaller by an order of magnitude, and the volume is larger by an order of magnitude, so the typical pressure can be smaller by two orders of magnitude.