how to efficiently select the input parameters for the targeted bands

[haiyuanwang2020]

Dear Lucy,

I hope this message finds you well.

I have a small question while I am using effmass. Please see the attached files.
I actually would like to estimate the effective mass for the band of VBM and CBM. A case is attached for Cs2AgBiBr6, which has the indirect band gap nature. I feel it is a bit difficult for me to get the suitable input parameters for my interested bands.

Please see the figure named "0_0.7_0.5.png", which is corresponding to the input below:
How many k-points should I ignore at the start of the file? (useful for hybrid calculations) [0]
How far (in eV) from the CBM (VBM) would you like me to search for minima (maxima)? 0.7
What would you like the energy range (in eV) of each segment to be? 0.5

The figure named "0_1_0.5.png" is corresponding to:
How many k-points should I ignore at the start of the file? (useful for hybrid calculations) 0
How far (in eV) from the CBM (VBM) would you like me to search for minima (maxima)? 1
What would you like the energy range (in eV) of each segment to be? 0.5

I played quite a lot of combinations about those parameters not just the two cased I showed you. It seems effmass is always trying to avoid the VBM without considering its effective mass. Do you have some suggestions how we can select the input parameters for the targeted bands? I am generally interested in the VBM and CBM region.

Many thanks for your time and looking forward to hearing from you.

Cs2AgBiBr6.zip

Best regards,
Haiyuan Wang.

[lucydot]

Hi @haiyuanwang2020,

Thanks for clear explanation of your problem and images sent.
Looking at the image 0_0.7_0.5.png it looks to me that the band is quite flat at the CBM (I think you have CBM and VBM mixed up on the image you sent and in text above? - CBM is the conduction band minimum so will be in the bands energetically above the band gap.)

If there are two points equal in energy at the CBM then effmass will not recognise this as a minimum point, as it does not have two points surrounding it which are higher in energy. Furthermore, the curvature in the "flat" direction would be zero, leading to an infinite effective mass (if we use the definition of effective mass being inverse of second derivative).

As a workaround, rather than using the command line interface as you have done you can use programming to specify exactly which k-points and bands you want effmass to investigate. There are some instructions for this here: #53. If you have not used Python before, then there may be a bit of a learning curve and I suggest seeking out someone in your group or institution to support you with this. If it is still unclear, then please do ask me questions here (and if I do not respond, feel free to prompt me via email!)

[haiyuanwang2020]

Dear Lucy,

Many thanks for your reply. Yes, that's a typo for VBM and CBM I marked in the figure.

Thank you for your explanation. I will try to use programming to specify them then.

Best regards,
Haiyuan.