Van der Waals exchange-correlation functionals and their high pressure and warm dense matter applications

We investigate basic hydrogen quantities like the molecular bond length, the molecular dissociation energy, and the van der Waals interaction in idealized situations in an effort to discern a suitable exchange-correlation functional for the molecular to metal transition in warm dense hydrogen. The best reproduction of bond length and dissociation energy is given by the r2SCAN functional. Several van der Waals functionals and also HSE06 fare qualitatively and quantitatively no better than Perdew–Burke–Ernzerhof or worse. r2SCAN however fails at reproducing the van der Waals energy minimum. In addition, we investigate quantities like the static and dynamic ion structure factor, and the electronic DOS to determine differences between exchange-correlation functionals with and without van der Waals corrections in the transition region from the molecular to the metallic regime of hydrogen.