How likely is it that metallic hydrogen is metastable at atmospheric pressure? I just watched a news report about the laboratory production of [solid] metallic hydrogen for the first time. I was surprised to find the researchers predicting that, once produced, it might remain solid at atmospheric pressure. The video says they will test this in a couple of weeks, so I guess we'll know then, but in the mean time I'm curious about how plausible this is.
As noted in the answer to a previous question it seems this prediction was made in the 1970s, and it seems that people have speculated since then about possible applications if it is the case. (For example, it would be a revolutionary rocket fuel.) However, it isn't clear to me whether that early result has been backed up or made less plausible by more recent calculations. Does anyone know the current state of the art in terms of predictions about the stability of metallic hydrogen at low pressures?
 A: I'm not aware of any modern studies that predict solid metallic hydrogen to be stable. In general, the metastable recovery of high-pressure phases all the way back down to atmospheric pressure is very rare. The most well known exception is the graphite-to-diamond phase transition, but in that case the energy difference between those two phases is only about 0.003 eV per atom. Don't know offhand what the energy difference per atom between hydrogen gas at one atmosphere and solid metallic hydrogen is, but you can be sure that it is enormous by comparison. Not saying that metastable metallic hydrogen is impossible, but it certainly would be counter to all expectations based on past experience with high-pressure phases of various materials.
(Side Note: I work in high-pressure physics and the creation of solid metallic hydrogen has long been the "Holy Grail" of the high-pressure community. This is not the first time that someone has claimed to have created solid metallic hydrogen by static compression, and the claim is currently getting a lot of scrutiny by others in the high pressure community. BTW, fluid metallic hydrogen has been created in the laboratory using reverberating shock waves. )
