  Electronic structure calculations on the low dimensional spin-1/2 compound
TiOCl were performed at several pressures in the orthorhombic phase, finding
that the structure is quasi-one-dimensional. The Ti3+ (d1) ions have one t2g
orbital occupied (dyz) with a large hopping integral along the b direction of
the crystal. The most important magnetic coupling is Ti-Ti along the b axis.
The transition temperature (Tc) has a linear evolution with pressure, and at
about 10 GPa this Tc is close to room temperature, leading to a room
temperature spin-Peierls insulator-insulator transition, with an important
reduction of the charge gap in agreement with the experiment. On the
high-pressure monoclinic phase, TiOCl presents two possible dimerized
structures, with a long or short dimerization. Long dimerized state occurs
above 15 GPa, and below this pressure the short dimerized structure is the more
stable phase.
