We present comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$: monoclinic $M_2$ and triclinic. The main result of our work is that the energy gap and the electronic structure are essentially unaltered by the first-order structural phase transition between the $M_2$ and triclinic phases. Moreover, the optical interband features in the $M_2$ and triclinic phases are remarkably similar to those observed in the well-studied monoclinic $M_1$ insulating phase of $\mathrm{V}{\mathrm{O}}_2$. As the energy gap is insensitive to the different lattice structures of the three insulating phases, we rule out vanadium-vanadium pairing (the Peierls component) as the dominant contributor to the opening of the gap. Rather, the energy gap arises primarily from intra-atomic Coulomb correlations.

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• Band gap
• Dielectric properties
• Metal-insulator transition
• Hybrid functionals
• Infrared spectroscopy
• Optical spectroscopy