The role of dispersion forces on the atomic structure of glassy chalcogenides: The case of GeSe4 and GeS4

Recent first-principles molecular dynamics simulations (FPMD) results on two chalcogenide glasses (glassy GeS4 and GeSe4) are revisited by accounting explicitly for van der Waals (vdW) dispersion forces. This effort is motivated by the observation that such contributions were found to be important in the case of glassy GeTe4 while they were negligible for another disordered chalcogenide system, liquid GeSe2. With the present results, we provide additional evidence intended to establish under which conditions and for which systems the consideration of dispersion forces plays a role in determining the atomic structure. For these purposes we employ two different dispersion schemes used in conjunction with the BLYP (Becke, Lee, Yang and Parr) exchange-correlation functional. The first is the DFT-D2 vdW correction introduced by Grimme et al. (Grimme (2006) [1]) while the second is the vdWW approach based on the Wannier functions analysis (Silvestrelli, 2008 [2]). The atomic structures obtained agree well with the ones obtained by Bouzid et al. [3] without considering dispersion forces. Due to the vdW interactions, glassy GeSe4 features a higher number of Ge fourfold coordinated. The two vdW approaches also agree to a large extent, exceptions occurring for some moderate differences in the intensity of the peaks in the Ge-Ge pair correlation function.