Yambo Community Forum

Dear forum,

I am doing convergence tests to calculate the GW band structure. The convergences related to RandQpts,RandGvec,EXXRLvcs and BndsRnXp were well done, but in the NGsBlkXp convergence , as seen in the attached figures, with increasing the value of NGsBlkXp for BndsRnXp = 90, the VBM starts to decrease again after reaching the convergence, and as a result, the gap also starts to increase. What is the cause of this behavior? Do I still have to do the calculation for NGsBlkXp greater values to achieve convergence?
In calculating the correct GW band structure, the gap is the criterion for convergence, or should convergence be checked relative to VBM and CBM?

Thanks,
Mitra Helmi

Dear Mitra,

there is no criterion to argue that the convergence should be monotone, so yes, you should keep increasing the NGsBlkXp value.
The gap energy is a good criterion if you are interested in optical properties, while you should look at the single bands if you need to converge them separately e.g. band alignments etc.

Please note also that NGsBlkXp and BndsRnXp are not dependent variable, ie if NGsBlkXp converge for a BndsRnXp value you may need larger value for NGsBlkXp when increasing the number of bands.

Best,
Daniele

Dear Daniele

In these convergence tests, I set the NGsBlkXp in the Ry at the inputs, but when I checked the value in RL at the output, the equivalent values are different from what is given in the Yambo site tutorial. For example, in the tutorial up to NGsBlkXp= 4Ry, the size of the dielectric matrix has not yet reached 40RL, but in the output of my calculations, the size of the dielectric matrix in NGsBlkXp = 4Ry has been reported to be 139RL. What is the reason for this difference?
Is this conversion dependent on the type of material?


Best regards,
Mitra Helmi

Dear Miytra,

when you provide the size of the matrix in Ry, ie using an energy cutoff, this means all the G's such that |G|^2/2 < Ecut
The value of the G depends on the geometry and size of your unit cell being the G_i=2pi*n/L_i ;(i=x,y,z)

Best,

Daniele

Dear Daniele

According to the attached figure, is it correct to consider the values of NGsBlkXp = 10 and BndsRnXp = 70 as convergent values, or should higher values be considered?

Best,
Mitra

Dear Mitra,

of course it depends on the accuracy you need. It seems to me pretty converged. You can try to extrapolate the last points with a reasonable function e.g. a+1/Ngblk and see if the fit is reasonable and estimate your error wrt the asymptotic value.

Best,
Daniele