I am performing DFT simulation (with Quantum Espresso) for a large number of crystalline materials to estimate their band gap energies. I am getting negative band gap values for many of them (difference between the lowest unoccupied level and the highest occupied level is negative). While I am aware that this is because of overlapping valence and conduction bands which is the case for metals, this also occurs for nonmetals with nonzero ground truth band gap energies. Could this be attributed to the underestimation issue of DFT methods in estimating the band gaps as highlighted by many studies?
An example of the output produced by Quantum Espresso for a crystal.
highest occupied, lowest unoccupied level (ev): 8.9114 8.5647
As a follow-up, I would also like to know if the number of bands of a crystal, which is an input parameter to DFT (
nbnd in Quantum Espresso's
pw.x) can be determined analytically for any material. At the moment, I am just using a high value of 256 for all crystals - it works in most cases when compared to lower values. So I was wondering if DFT is very sensitive to this parameter and could lead to more incorrect and negative band gaps if an inexact number of bands is provided.