A new criterion of accuracy of <i>ab initio </i>calculations of ferroelectrics and reexamination of stain-enhanced ferroelectricity

In the ab initio calculations of ferrroelectrics (FE), some exchange-correlation (XC) energy functionals such as LDA are considered to show good agreement with experiments at RT, e.g., spontaneous polarization P_S, as compared with other XC functionals. This is due to the error compensation of the RT effect and, hence, will be ineffective in other situations, e.g. domain boundaries. Here, FEs under large strain at RT are approximated as those at 0 K.
To find effective XC functionals without error compensation of RT, we propose a new criterion: properties at 0 K by the Ginzburg-Landau (GL) theory, because GL theory is a thermodynamic description of experiments under the same symmetry-constraints as ab initio calculations. This criterion reveals the high accuracy of some XC functionals superior to XC functionals that have been regarded as accurate.
For example, we propose a correct value of ab initio P_S under strain-free condition is ~32 μC/cm² for BaTiO₃ instead of conventional 26 μC/cm² and > 84 μC/cm² for PbTiO₃ instead of conventional 70 μC/cm². The validity is confirmined by the calculations of strained BaTiO₃. In addition, much more enhanced FE by strain than the previous reports is found.
Y. Watanabe, J. Chem. Phys.148, 194702 (2018)