van der Waals interaction in iron-chalcogenide superconductors

We demonstrate that the inclusion of van der Waals dispersive interaction sensibly improves the prediction of lattice constants by density functional theory in iron-chalcogenides (FeCh) superconductor compounds, namely FeSe and FeTe. We show how generalized gradient approximation (GGA) for the exchange correlation potential overestimates the out-of-plane lattice constants in both compounds when compared with experiments. In addition, GGA predicts a too weak bonding between the neutral FeCh layers, with a sensible underestimation of the bulk modulus. van der Waals corrected simulations completely solve both problems, reconciling theoretical results with experiments. These findings must be considered when dealing with theoretical predictions in FeCh compounds.