  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.
