Theoretical investigation of the pressure-induced metallization and the collapse of the antiferromagnetic state ofNiI2

${\mathrm{NiI}}_2$ undergoes an isostructural transition under pressure from an insulating antiferromagnetic (AF) state to a metallic but nonmagnetic (NM) state. We investigate this transition with the full-potential linearized augmented-plane-wave method within the density-functional theory, where we use both the local-spin-density approximation and the generalized gradient approximation. We optimized the internal coordinate u under pressure, which defines the I position, and show that the I-layer/I-layer distance shrinks drastically, while the I-layer/Ni-layer distance remains almost constant. At zero pressure the AF state is lower in energy but the situation changes under pressure and the NM state becomes the ground state. To illustrate this transition we show the pressure dependence of the hyperfine field. We investigate the insulator-metal transition, analyze the bonding between Ni and I and find a strong covalent contribution resulting in a small band gap between the occupied antibonding I 5p and Ni 3d-${\mathit{e}}_{\mathit{g}}$ spin-up states and the corresponding unoccupied spin-down states. This gap vanishes under pressure.