Abstract
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(r) itself. An accurate description of n(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 PbZrO3 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.
Highlights
The ABO3 perovskite oxides are well known for their vast and rich variety of physical phenomena
Calculations using the plane-wave basis set are performed with the ABINIT code [47, 48] (v8.10.2) whilst calculations utilising pseudoatomic orbital (PAO) are carried out using the CONQUEST code (v1.0) [27, 32] with the direct diagonalisation of the Hamiltonian matrix
We have investigated the consequences of representing delicate features of the perovskite oxides with the default CONQUEST basis sets of PAOs as a replacement for planewaves
Summary
The ABO3 perovskite oxides are well known for their vast and rich variety of physical phenomena. Of the simulation cell which introduces wasteful calculations on the grid for systems including a vacuum region These issues can be bypassed by replacing plane-waves with physically intuitive local basis sets of pseudoatomic orbitals (PAOs) [19, 20, 21, 22, 23]. Complete with a change in algorithm (the scope of which is beyond this work but discussed in references [30, 27]), this allows the well known O(N 3) scaling wall (where N is the number of atoms in the simulation) in standard DFT to be broken and replaced with a code which scales as O(N ) This method paves the way for full electronic structure calculations on systems of many thousands of atoms (or even millions [31]), well beyond what is possible with conventional plane-wave methods. This includes a discussion of the impact this work has on the topic of local basis sets and the promise of accurate and large-scale electronic structure calculations on the perovskite oxides
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