Role of the van der Waals interactions and impact of the exchange-correlation functional in determining the structure of glassy GeTe4

The structural properties of amorphous GeTe4 are studied within the framework of first-principles molecular dynamics combined with density functional theory. Four different theoretical schemes are considered, each one intended to correspond to a distinct structural model. These are obtained by selecting either the PBE (Perdew, Burke, and Ernzerhof) or the BLYP (Becke, Lee, Yang, and Parr) exchange-correlation functionals and, for each one of the two, by disregarding or including van der Waals dispersion forces. Based on the comparison with experimental total structure factor S(k) and total pair correlation function g(r), one can infer the quantitative character of such models, thereby extracting information on the underlying structures. We found that the inclusion of the vdW forces improves the predictive power of our approach for the PBE and (to a smaller extent) the BLYP exchange-correlation functionals. Overall, BLYP performs better than PBE in reproducing the available experimental quantities, providing a tetrahedral atomic-scale network profoundly different from the one predicted by PBE, in which tetrahedral and octahedral motifs do coexist. Our work demonstrates that a careful choice of the exchange-correlation functional, combined with the account of van der Waals forces, is crucial to achieve realistic structural predictions for glassy GeTe4.