  Structural modelling of octahedral tilts in perovskites is typically done
using the symmetry constraints of the resulting space group. In most cases,
this introduces more degrees of freedom than those strictly necessary to
describe only the octahedral tilts. It can therefore be a challenge to
disentangle the octahedral tilts from other structural distortions such as
cation displacements and octahedral distortions. This paper reports on the
development of constraints for modelling pure octahedral tilts and implemented
the constraints in diffpy-CMI, a powerful package to analyse pair distribution
function (PDF) data. The program allows features in the PDF that come from
rigid tilts to be separated from non-rigid relaxations, provides an intuitive
picture of the tilting, and as it has many fewer refinable variables than the
unconstrained space-group fits, provides robust and stable refinements of the
tilt components. It further demonstrates the use of the model on the canonical
tilted perovskite CaTiO$_3$ which has a known Glazer tilt system $\alpha^+
\beta^- \beta^-$. The Glazer model fits comparably to the corresponding space
group model $Pnma$ below $r$ = 14 A and becomes progressively worse than the
space group model at higher $r$ due to non-rigid distortions in the real
material.
