  The perovskite oxides are known to be susceptible to structural distortions
over a long wavelength when compared to their parent cubic structures. From an
ab initio simulation perspective, this requires accurate calculations including
many thousands of atoms; a task well beyond the remit of traditional plane
wave-based density functional theory (DFT). We suggest that this void can be
filled using the methodology implemented in the large-scale DFT code, CONQUEST,
using a local pseudoatomic orbital (PAO) basis. Whilst this basis has been
tested before for some structural and energetic properties, none have treated
the most fundamental quantity to the theory, the charge density $n(\mathbf{r})$
itself. An accurate description of $n(\mathbf{r})$ is vital to the perovskite
oxides due to the crucial role played by short-range restoring forces
(characterised by bond covalency) and long range coulomb forces as suggested by
the soft-mode theory of Cochran and Anderson. We find that modestly sized basis
sets of PAOs can reproduce the plane-wave charge density to a total integrated
error of better than 0.5% and provide Bader partitioned ionic charges, volumes
and average charge densities to similar degree of accuracy. Further, the
multi-mode antiferroelectric distortion of PbZrO$_3$ and its associated
energetics are reproduced by better than 99% when compared to plane-waves. This
work suggests that electronic structure calculations using efficient and
compact basis sets of pseudoatomic orbitals can achieve the same accuracy as
high cutoff energy plane-wave calculations. When paired with the CONQUEST code,
calculations with high electronic and structural accuracy can now be performed
on many thousands of atoms, even on systems as delicate as the perovskite
oxides.
