### a question for cofactor in function ip_eps2

Posted:

**Sat Sep 14, 2024 2:14 am**Dear developer,

I am confused about the cofactor used in the function ip_eps2 in dipolesdb.py in yambopy : https://github.com/yambo-code/yambopy/b ... polesdb.py

In line 381 the cofactor was defined as
I am curious about why the volume used here is the Recip. lattice volume.

In the equation used to calculate the epsilon or absorption coefficient, it seems that the volume of the lattice cell (with the unit of m^3 or other cubic length) but not that of recip lattice was used.

For example, equation 10 in [PWSCF s epsilon.x users manual] https://web.mit.edu/espresso_v6.1/amd64 ... ps_man.pdf

I have this question because the values of the imaginary part of epsilon calculated by ip_eps2 in yambopy is much larger than that given by "yambo -o c", although they have the same shape.

However, if I used the Direct lattice volume in the cofactor (i.e., spin_deg*8.*np.pi/(self.lattice.lat_vol)), I got valueS much smaller than those calculated by "yambo -o c".

I found that the values of the imaginary part of epsilon calculated by a cofactor of spin_deg*8.*np.pi/(self.lattice.rlat_vol*self.lattice.lat_vol) matched well with that given by "yambo -o c" in both the sample and values. But this definition of cofactor causes more confusion.

Thank you very much for your kind help!

Zhipeng

Tongji University, Shanghai, China

I am confused about the cofactor used in the function ip_eps2 in dipolesdb.py in yambopy : https://github.com/yambo-code/yambopy/b ... polesdb.py

In line 381 the cofactor was defined as

Code: Select all

```
cofactor = spin_deg*8.*np.pi/(self.lattice.rlat_vol)
```

In the equation used to calculate the epsilon or absorption coefficient, it seems that the volume of the lattice cell (with the unit of m^3 or other cubic length) but not that of recip lattice was used.

For example, equation 10 in [PWSCF s epsilon.x users manual] https://web.mit.edu/espresso_v6.1/amd64 ... ps_man.pdf

I have this question because the values of the imaginary part of epsilon calculated by ip_eps2 in yambopy is much larger than that given by "yambo -o c", although they have the same shape.

However, if I used the Direct lattice volume in the cofactor (i.e., spin_deg*8.*np.pi/(self.lattice.lat_vol)), I got valueS much smaller than those calculated by "yambo -o c".

I found that the values of the imaginary part of epsilon calculated by a cofactor of spin_deg*8.*np.pi/(self.lattice.rlat_vol*self.lattice.lat_vol) matched well with that given by "yambo -o c" in both the sample and values. But this definition of cofactor causes more confusion.

Thank you very much for your kind help!

Zhipeng

Tongji University, Shanghai, China