How do molecules behave at extremely high pressure?

Roald Hoffman in American Scientist:

ScreenHunter_02 Mar. 08 18.44 Scientists love subjecting matter to extreme conditions. And the variable of pressure, at its high end, is perhaps the most interesting one to explore for both chemistry and physics. For although we can estimate the (very short) lifetimes of molecules at temperatures of the sun, and what chemistry might transpire at a nanokelvin or in a vacuum “higher” than that of outer space, the realm of high pressure, such as that at the center of a planet, gives us pause.

The behavior of matter under high pressure is just not obvious, and this makes it fun to explore. No, it’s not sadism, just curiosity. Other motivations? It’s impossible to probe directly the core of Earth or Saturn; could we do it in the lab or on a computer? Also, predicting the behavior of matter under extreme conditions is a great test of whether we really do understand what’s going on.

Let me tell you about some remarkable goings on in the world of high pressure.

More here.