Dynamic electronic correlation effects in NbO2 as compared to VO2

In this paper we present a comparative investigation of the electronic structures of ${\mathrm{NbO}}_2$ and ${\mathrm{VO}}_2$ obtained within a combination of density functional theory and cluster-dynamical mean-field theory calculations. We investigate the role of dynamic electronic correlations on the electronic structure of the metallic and insulating phases of ${\mathrm{NbO}}_2$ and ${\mathrm{VO}}_2$, with a focus on the mechanism responsible for the gap opening in the insulating phases. For the rutile metallic phases of both oxides, we obtain that electronic correlations lead to a strong renormalization of the $t_{2g}$ subbands, as well as the emergence of incoherent Hubbard subbands, signaling that electronic correlations are also important in the metallic phase of ${\mathrm{NbO}}_2$. Interestingly, we find that nonlocal dynamic correlations do play a role in the gap formation of the [body-centered-tetragonal (bct)] insulating phase of ${\mathrm{NbO}}_2$, by a similar physical mechanism as that recently proposed by us in the case of the monoclinic ($M_1$) dimerized phase of ${\mathrm{VO}}_2$ [ Phys. Rev. Lett. 117, 056402 (2016)]. Although the effect of nonlocal dynamic correlations in the gap opening of bct phase is less important than in the ($M_1$ and $M_2$) monoclinic phases of ${\mathrm{VO}}_2$, their presence indicates that the former is not a purely Peierls-type insulator, as it was recently proposed.