[QE-users] Silicon gap vs. temperature

[David Fernandes Machado]

Hello all,
 
I am trying to determine the dependence of the energy gap of silicon as a function of temperature. In the literature, it is stated that the decrease in the energy gap of silicon with increasing temperature can be explained by thermal expansion and electron-phonon interaction.
 
First, I used the thermo_pw library (which uses the QHA approximation) to determine the lattice parameter of silicon as a function of temperature. Then, I ran the following calculations: SCF, NSCF, DOS, band, and finally plotband. I performed these calculations using the lattice parameters of Si corresponding to temperatures in a range from 4K to 800K. For this simulation, I am using PBE pseudopotentials, an ecutwfc of 25 Ry, and a unit cell with 2 atoms.
 
The problem is that the gap increases with temperature instead of decreasing. I obtained a gap of 0.6187 eV at 4K and 0.6315 eV at 800K.
 
I also tried calculating the band structure considering electron-phonon coupling using the EPW library, but the gap still increases with temperature.
 
Has anyone already tried to calculate the silicon gap as a function of temperature? What am I doing wrong?
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