Effects of model size in density-functional-theory study of alloys: A case study of CsPbBr$_2$Cl

TL;DR

This study investigates how the size of computational models affects the accuracy of density-functional-theory calculations for alloy systems, specifically using CsPbBr$_2$Cl, highlighting the importance of model size in capturing chemical disorder and entropy effects.

Contribution

It demonstrates that larger models provide a more accurate representation of alloy formation energy distributions and entropy stability, addressing a key challenge in DFT studies of alloys.

Findings

Larger models yield narrower formation energy distributions.

Distribution of Br parameters is narrower in bigger models.

Entropy stability effects are more pronounced at high temperatures with larger models.

Abstract

The primary challenge of density-functional-theory exploration of alloy systems concerns the size of computational model. Small alloy models can hardly exhibit the chemical disorder properly, while large models induce difficulty in sampling the alignments within the massive material space. We study this problem with the {\gamma} phase of the mixed halide inorganic perovskite alloy CsPbBr$_2$Cl. The distribution of alloy formation energy becomes narrower when the size of the model system increases along $\sqrt{2}\times\sqrt{2}\times2$, $2\times2\times2$, and $2\sqrt{2}\times2\sqrt{2}\times2$ models. This is primarily because the distribution of Br distribution parameters, which plays a leading role in determining the formation energy range, is more narrow for larger models. As a result, larger entropy stability effect can be observed with larger models especially at high temperatures, for which the approximation using mixing entropy based on the ideal solution model becomes better.