Symmetry breaking in vanadium trihalides

In the light of new experimental evidence we study the insulating ground state of the 3d2 -transition metal trihalides VX 3 (X = Cl, I). Based on density functional theory with the Hubbard correction we systematically show how these systems host multiple metastable states characterised by different orbital ordering and electronic behaviour. Our calculations reveal the importance of imposing a precondition in the on site d density matrix and of considering a symmetry broken unit cell to correctly take into account the correlation effects in a mean field framework. Furthermore we ultimately found a ground state with the a1g orbital occupied in a distorted VX 6 octahedra driven by an optical phonon mode.