Hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxides

TL;DR

This paper evaluates various DFT and hybrid functionals for ferroelectric oxides, finds limitations in existing functionals, and proposes a new hybrid functional that accurately predicts both electronic and structural properties.

Contribution

The paper introduces a new hybrid exchange-correlation functional that improves the accuracy of electronic and structural property predictions for ferroelectric oxides.

Findings

Existing functionals fail to accurately predict both electronic and structural properties simultaneously.

Common hybrid functionals improve band gap predictions but overestimate ferroelectric distortions.

The proposed functional provides balanced accuracy for electronic and structural properties.

Abstract

Using a linear combination of atomic orbitals approach, we report a systematic comparison of various Density Functional Theory (DFT) and hybrid exchange-correlation functionals for the prediction of the electronic and structural properties of prototypical ferroelectric oxides. It is found that none of the available functionals is able to provide, at the same time, accurate electronic and structural properties of the cubic and tetragonal phases of BaTiO$_3$ and PbTiO$_3$. Some, although not all, usual DFT functionals predict the structure with acceptable accuracy, but always underestimate the electronic band gaps. Conversely, common hybrid functionals yield an improved description of the band gaps, but overestimate the volume and atomic distortions associated to ferroelectricity, giving rise to an unacceptably large $c/a$ ratio for the tetragonal phases of both compounds. This super-tetragonality is found to be induced mainly by the exchange energy corresponding to the Generalized Gradient Approximation (GGA) and, to a lesser extent, by the exact exchange term of the hybrid functional. We thus propose an alternative functional that mixes exact exchange with the recently proposed GGA of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] which, for solids, improves over the treatment of exchange of the most usual GGA's. The new functional renders an accurate description of both the structural and electronic properties of typical ferroelectric oxides.