# Re: [Wien] degeneracy problem in dipole matrix elements

```Dear Mengxi Wu

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If I clearly understand you, you are considering the GAMMA point where the bands are degenerate.
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At the GAMMA point these bands have the same symmetry to my point of view, explaining why the dipole matrix is non-zero. Thus your result is expected to my point of view.
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Best Regards

Xavier

Le 13/11/2016 à 17:29, Little Grass a écrit :
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```Dear WIEN2k users,

```
I'm trying to study the dipole matrix elements of MgO at Gamma point, but the dipole matrix elements between the degenerated bands seem to have been mixed together. Are there any ways to sort out the degeneracy, so that the dipole matrix elements are non-zero only for bands with the same symmetry?
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```
The band structure can be generated from the spaghetti program. In addition, bands with different symmetries are colored differently with irrep. For example, Fig1 shows the band structure from X to Gamma to X. According to symmetry, only the bands with the same color will be dipole allowed in px. So we expect that the dipole matrix elements (in file mommat2) for px is zero between bands with different colors. For example, the dipole matrix elements between band 6 and band 9 is non-zero, but the dipole matrix elements between band 7 and 9 or 8 and 9 are zero. But if one looks at the px component of the dipole matrix element from the optic program, we see that at Gamma point, the dipole matrix elements between the degenerated bands are mixed together. For example, dipole matrix elements between band 6 and 9, between 7 and 9, and between 8 and 9 are all non-zero. In other words, the dipole matrix is much denser than it should be. So is there a way to fix the dipole matrix so that the dipole matrix become very sparse and only bands with the same symmetry have non-zero dipole matrix elements?
```
Fig1:http://imgur.com/a/c7p0u

Thanks in advance for the help.
Best,
Mengxi Wu

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