Ground-state properties of rutile: electron-correlation effects
  Electron-correlation effects on cohesive energy, lattice constant and bulk compressibility of rutile are calculated using an ab-initio scheme. A competition between the two groups of partially covalent Ti-O bonds is the reason that the correlation energy does not change linearly with deviations from the equilibrium geometry, but is dominated by quadratic terms instead. As a consequence, the Hartree-Fock lattice constants are close to the experimental ones, while the compressibility is strongly renormalized by electronic correlations. 
Krzysztof Rosciszewski, Klaus Doll, Beate Paulus, Peter Fulde, Hermann   Stoll