  We use density functional theory to explore the interplay between octahedral
rotations and ferroelectricity in the model compound SrTiO$_3$. We find that
over the experimentally relevant range octahedral rotations suppress
ferroelectricity as is generally assumed in the literature. Somewhat
surprisingly we observe that at larger angles the previously weakened
ferroelectric instability strengthens significantly. By analysing geometry
changes, energetics, force constants and charges, we explain the mechanisms
behind this transition from competition to cooperation with increasing
octahedral rotation angle.
