Yep. Personally, I trust DFT about as far as I can throw it, and think that spending time worrying about functionals is fairly pointless and likely to end up over-fitting data, especially if they aren't incorporating new physics :)
That last paper does say, however:
> We used the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation density functional, which is well suited for very high-pressure studies, as the charge density is more uniform than at low densities, and it obeys the uniform limit and gives a good account of the linear response of the electron gas to an external potential
but also
> DFT studies of high-pressure phases of hydrogen have been performed using several approximate density functionals, and a significant dependence of the results on the functional has been noted. The enthalpy differences between phases are so small that changes of only a few meV per proton can make a noticeable difference to the phase diagram
so that's some physics. They seem to end up using DMC for the static lattice and DFT for the vibrational corrections on top of that.
That last paper does say, however:
> We used the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation density functional, which is well suited for very high-pressure studies, as the charge density is more uniform than at low densities, and it obeys the uniform limit and gives a good account of the linear response of the electron gas to an external potential
but also
> DFT studies of high-pressure phases of hydrogen have been performed using several approximate density functionals, and a significant dependence of the results on the functional has been noted. The enthalpy differences between phases are so small that changes of only a few meV per proton can make a noticeable difference to the phase diagram
so that's some physics. They seem to end up using DMC for the static lattice and DFT for the vibrational corrections on top of that.