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

The pseudoatomic orbital basis: electronic accuracy and soft-mode distortions in ABO$_3$ perovskites