# The "Tetrahedron method does not include variations of the Fermi occupations" Problem

I am doing computation of tetragonal CsPbBr3 perovskites. When I set ISMEAR = -5 I found such a problem:

This problem is absent at ISMEAR = 0. What is the cause of the problem and how do I solve it?

• Don't use ISMEAR = -5 for a relaxation. That's your problem. Jun 12, 2023 at 2:09
• But I didn't. That's a static run.
– 蕭力諶
Jun 12, 2023 at 4:43

The issue has been resolved. It seems that it was caused by rounding the cell parameters to the sixth digit after decimal point instead of printing all 16 digits of it. (I was applying some strain and rounded it, never thought that such subtle differences would make such a huge impact)

An explanation is available in the VASP manual

For the calculation of the total energy in bulk materials, we recommend the tetrahedron method with Blöchl corrections (ISMEAR=-5). This method also gives a good account of the electronic density of states (DOS). The only drawback is that the method is not variational with respect to the partial occupancies. Therefore the calculated forces and the stress tensor can be wrong by up to 5 to 10% for metals. Only for semiconductors and insulators, the forces are correct because the partial occupancies do not vary and are either zero or one.

The manual goes in to say how to modify these parameters for metals if you want accurate forces, but your system appears to be a semiconductor.

• AwakenYesterday, was it you that downvoted the other answer? Jun 12, 2023 at 15:21

The system is trying to tell you that your choices of the parameters is PHYSICALLY nonsense. You have to learn to choose sensible things to do.

DFT is not meant to be learnt solely by yourself. Please get instructional help from people who actually know what they are doing. This behaviour is by design—there was a physics package that was aimed to be too easy to use, and then people started publishing a lot of nonsense by misusing the package, and that led to nobody being able to trust the papers citing that package. The entire package had to be scrapped.

• What physics package you are talking about? Do you remember the name? I find the story interesting. Jun 11, 2023 at 22:52
• Vienna Ab-initio Simulation Package?
– 蕭力諶
Jun 12, 2023 at 0:52
• @蕭力諶 no, it is not VASP. I meant to ask which physics package had to be scrapped due to people publishing lots of nonsense. Jun 12, 2023 at 2:21