[SOLVED] Probable bug in the ddk calculation or optic

[Robin]

Dear developers,

As I mentioned earlier (viewtopic.php?f=10&t=1153), I found that ABINIT (both 6.2.2 and 6.8.1) gives wrong linear optical spectrum in several 1D systems by using the usual procedures. Here I elaborate this issue with a simple example of carbon chain.

The known procedures to get linear optical spectrum are as follows: (1) SCF calculation to get the electron density of ground state; (2) NSCF calculation to get wavefunction with denser k-point grid and more empty bands; (3) ddk calculation to get the derivatives of wavefunction with respect to k, and then using optic program to get the dielectric function.

In the case of carbon chain system, I find that the (2)-step must be SCF to get correct optical results, which are confirmed by calculations of YAMBO and CASTEP. To simplify the explanation, I did the following tests: (a) generate the KSS by using ABINIT, which also outputs WFK; (b) calculate the ddk and spectrum by using both ABINIT and YAMBO. I find that YAMBO gives correct spectrum whether the (a)-step is NSCF or SCF, while ABINIT give wrong result when the (a)-step is NSCF. This clearly shows that there must be some problem. Test files and comparison graph are attached for your confirmation.

An interesting phenomenon possibly indicates the cause: only 1 SCF step in (a) is enough for ABINIT to get correct result, though I also tried 1000 steps! Maybe the ddk calculation or optic analysis does get correct information from a NSCF wavefunction.

Because this ddk calculation and optic analysis are relevant to several other RF calculations, like IR and Raman, this issue really matters. I am looking forward to detail analyses from you.

Sincerely,
Guangfu Luo


P.S. I checked the file-header of the NSCF and SCF WFK and did find differences: in the NSCF WFK, there are 2 fully occupied bands (4 electrons) according to “occ”, while in the SCF WFK, there are 3 fully occupied bands (6 electrons). The unit cell contains only 1 carbon atom.

[mverstra]

Hello Guangfu,

the main difference between the scf and non scf wf files should be the occupation numbers. This is also linked to whether you use iscf = -2 or -3. Could you check the occupations you get in both cases for the optics run? Or just from the WF files with cut3d, check the header.

Matthieu

[jzwanzig]

Also, the DDK calculation requires only filled bands, no partially filled or empty bands.

[Robin]

Dear Matthieu and Zwanziger,

Thank you both for the information. As Matthieu pointed out, the occupations in SCF and NSCF WFK are different. After switching iscf=-2 to iscf=-3 (the later initializes occ and wtk), I got correct optical spectra again!

So the cause could be the incorrect band filling in the WFK from iscf=-2, which causes incorrect electron transition in the optic analysis?

Sincerely,
Guangfu Luo