Structural and vibrational properties of VO2 from DFT and DFT+U calculations*

Vanadium dioxide (VO$_2$) undergoes a metal-insulator transition (MIT) at 340\,K from a metallic, high-temperature rutile phase to a insulating, low-temperature monoclinic phase. In thin films, the extremely fast switching times ($\simeq 100$~femtoseconds) of the MIT have led to many suggested device applications. Understanding the MIT driving mechanism and the long-debated importance of electronic correlation is important to these developments. We have computed the relaxed geometry and phonon frequencies using DFT and DFT+U for both phases of VO$_2$. The dependence of vibrational mode frequencies and oscillator strengths on the Hubbard $U$ parameter and their sensitivity to the Born effective charges in the insulating monoclinic phase will be reported. The calculated frequencies for $U=5$ eV are in good agreement with recent experimental infrared micro-spectroscopy measurements on single crystal platelets of VO$_2$ \footnote{T. J. Huffman et al., PRB, submitted.}. Our results indicate that strong electron-electron correlation must be included to describe the vibrational properties.