A comparison of DFT methods: Density functional theory (DFT) is now routinely used for simulating material properties. Many software packages are available, which makes it challenging to know which are the best to use for a specific calculation. Lejaeghere et al. compared the calculated values for the equation of states for 71 elemental crystals from 15 different widely used DFT codes employing 40 different potentials (see the Perspective by Skylaris). Although there were variations in the calculated values, most recent codes and methods converged toward a single value, with errors comparable to those of experiment. Science , this issue p. 10.1126/science.aad3000 ; see also p. 1394

Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as $Z$-carbon, is formed by pure $s{p}^3$ bonds and it provides an explanation to several features in experimental x-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to $Z$-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that $Z$-carbon is a transparent wide band-gap semiconductor with a hardness comparable to diamond.

Due to its low atomic mass, hydrogen is the most promising element to search for high-temperature phononic superconductors. However, metallic phases of hydrogen are only expected at extreme...

We present state-of-the-art first-principles calculations of the electronic and optical properties of silicon allotropes with interesting characteristics for applications in thin-film solar cells. These new phases consist of distorted sp3 silicon networks and have a lower formation energy than other experimentally produced silicon phases. Some of these structures turned out to have quasidirect and dipole-allowed band gaps in the range 0.8\char21{}1.5 eV, and to display absorption coefficients comparable with those of chalcopyrites used in thin-film record solar cells.