VO₂: Orbital competition, magnetism, and phase stability

The relative phase stability of VO₂ is one of the most fundamental issues concerning the metal-insulator transition in this material but has been so far largely unexplored theoretically. We investigate the relative stability of various phases of VO₂ using different levels of energy functionals within density functional theory (DFT). It is found that straightforward applications of several popular energy functionals, including the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, result in a wrong prediction for the ground state of VO₂. In particular, although the HSE and DFT+U methods are able to produce a band gap in the M₁ phase, they strongly favor the formation of local magnetic moments, a result that clearly disagrees with experiments. We also examine the effect of the occupation and the redistribution of the d derived t_2g (i.e., d_xz, d_yz, and d_x2-y2) orbitals of V atoms on the calculated relative phase stability of VO₂. We find that a small change in d occupation can result in a drastically different theoretical prediction. With the introduction of an orbital-dependent potential, a complete separation between the d_x2-y2 derived valence band and d_xz and d _yz derived conduction bands in the M₁ phase is achieved, resulting in a slight redistribution of the d occupation and a more faithful account of the polarization of the t_2g orbitals. This slight rearrangement of the d occupation also leads to a relative phase stability of VO₂ (including structural and magnetic phases) that agrees well with experiment.