Mott transition and suppression of orbital fluctuations in orthorhombic 3$d^1$ perovskites

Using $t_{2g}$ Wannier-functions, a low-energy Hamiltonian is derived for orthorhombic $3d^1$ transition-metal oxides. Electronic correlations are treated with a new implementation of dynamical mean-field theory for non-cubic systems. Good agreement with photoemission data is obtained. The interplay of correlation effects and cation covalency (GdFeO${}_3$-type distortions) is found to suppress orbital fluctuations in LaTiO${}_3,$ and even more in YTiO${}_3$, and to favor the transition to the insulating state.