Closure of the charge-transfer energy gap and metallization ofNiI2under pressure

We have performed resistivity and near-ir absorption measurements on ${\mathrm{NiI}}_{2}$ under pressure that strongly suggests that closure of the charge-transfer energy gap is the fundamental cause of the insulator-to-metal transition in ${\mathrm{NiI}}_{2}$. Our findings support the framework proposed by Zaanen, Sawatsky, and Allen outlining the electronic structure of transition-metal compounds.