Negative energy in phonon band structure calculation

[Utpal Sarkar]

I am a beginner in quantum ATK and facing a problem during phonon band structure calculation I am getting negative energy states. Could you please suggest me how to overcome this?
I have re optimized the structure with high force and stress tolerance but unable to get the correct phonon band structure. I am attaching the inputs and outputs for your reference.

[NW]

Hi,

I have one suggestion that may help you. Consider bigger cell for phonon calculation.

[Utpal Sarkar]

What  do you mean by bigger cell? whether you want to mean that I have to design a  super cell and re-optimize it and followed by phonon band structure calculation or you want to mean that I have to increase the size of the lattice vector of the unit cell and re-optimize it and followed by phonon band structure calculation. Kindly enlighten me on this topic.

[NW]

I mean repeat the structure in x and y directions (2*2) and then re-optimize the cell and structure. Then calculate phonon bandstructure.

[Utpal Sarkar]

Sir as per your suggestion I have performed phonon band structure calculation for 2*2*1 super cell. But the system still have negative energy of -6.18 meV which is earlier -6.27 for unit cell. To further increase the super cell size is very difficult for computation. Could you please suggest any other way to solve this?
I am attaching here input and output for your reference.

[Troels-Markussen]

Hi, I noticed that in your geometry optimization, you fixed the strain in the x- and y- directions, while not in the z-direction. I think this is a mistake. For such a 2D material you should rather fix the strain in the z-direction and relax it in the x- and y-directions like:

Code

constraints = [FixStrain(x=False, y=False, z=True)]

Try to re-run the geometry optimization with this constraint and the recalculate the DynamicalMatrix. Additionally, phonon calculations might require more accurate setting than 'normal' calculations. For this you might consider decreasing the tolerance as:

Code

iteration_control_parameters = IterationControlParameters(
    tolerance=1e-05,
    )

Regards, Troels