Abstract: The study of transition metal oxide spinels represents a considerable challenge for theory, due to the richness of their properties and to their natural tendency to form ordered states upon phase transitions. MgTi2O4 for example undergoes a transition from a metallic to a spin-singlet insulator near Tc=260 K, with the opening of an optical gap and no sign of Drude weight. The metallic phase is characterized by a pyrochlore lattice occupied by Ti atoms. The transition to a tetragonal structure is accompanied by a selective shortening of some bonds under formation of 'molecular dimers'. In this talk we address the problem of the origin of structural distortion in this material, by performing state-of-art correlated band structure calculations within the LDA+ Hubbard U scheme. Therein we identify an orbital selective MIT that is carried by orbital rehybridization assisted by correlation, without full orbital polarization. Additionally, we derive the structural distortion by expressing the forces in the LDA+U potential and by performing Parrinello-Rahman structure relaxation. Along this line we find a lock-in into a dimerized phase for a critical U value. Reference: S.Leoni, A. Yaresko, N. Perkins, H. Rosner, L. Craco, submitted to PRL.