# Deformation constant for 2D materials

Dear Alex,
I hope you are well.I have two questions about the deformation potential.

1. The vasp code does not give correct CBM and VBM edges for 2D materials. For accurate CBM and VBM edges, we need to subtract the vacuum energy from these. Is this feature included in amset code?
2. The deformation potential describes the change in energy of the bands with a change in volume but for two-dimensional material but there is a large vacuum along the z-direction in 2D materials. Do we need to multiply the volume factor (V/monolayer thickness) to deformation constant?

Hi Aamir,

1. As far as I know, subtracting the vacuum energy from the CBM and VBM is used to obtain the electron affinity and ionisation potential relative to the vacuum level. I don’t think this is needed in our case as amset uses the absolute deformation potentials, i.e., how much the eigenvalues shift with strain. Accordingly, only the relative eigenvalues between calculations matters.
2. The actual formula for deformation potential used in amset relies on the shift in energies with strain rather than volume: D_{n\mathbf{k},\alpha\beta} = \delta \varepsilon_{n\mathbf{k}} / S_{\alpha\beta} where \mathbf{S} is the uniform stress tensor, \mathbf{D}_{n\mathbf{k}} is the deformation potential tensor at wave vector \mathbf{k} and band n, and \alpha and \beta are cartesian directions. As such, the presence of the vacuum should not impact the deformation calculation.

Best,
Alex

Dear Alex,
Thank you for your reply. What about the elastic constants? I think we should multiply the vacuum height to elastic constants.

4 posts were split to a new topic: What is the deformation potential reference energy