Features Reqeust: Fermi surface nesting function and bare static electron susceptibility

[houzf]

The Fermi surface nesting function $\xi_\mathbf{q}$ is defined as:

$$\xi_\mathbf{q} =\frac{1}{N}\sum_{\mathbf{k}, i, j}\delta(\epsilon_{\mathbf{k},i}-\epsilon_\mathrm{F})\delta(\epsilon_{\mathbf{k}+\mathbf{q},j}-\epsilon_\mathrm{F})$$

where $\epsilon_{\mathbf{k},I}$ is the Kohn-Sham eigenvalue and $i$, $j$ are the indices of energy bands, $N$ is the number of $\mathbf{k}$ points, and $\epsilon_\mathrm{F}$ is the Fermi energy.

The bare static electron susceptibility ($\chi_\mathbf{q}$) is also called bare Lindhard function and defined as below:

$$\chi_\mathbf{q} =-\frac{1}{N}\sum_{\mathbf{k}, i, j}\frac{f(\epsilon_{\mathbf{k},i}-\epsilon_\mathrm{F})-f(\epsilon_{\mathbf{k}+\mathbf{q},j}-\epsilon_\mathrm{F})}{\epsilon_{\mathbf{k},i}-\epsilon_{\mathbf{k}+\mathbf{q},j}}$$

where $f$ is the Fermi-Dirac function.

These defintions can be referred to

1. M. D. Johannes and I. I. Mazin, Fermi surface nesting and the origin of charge density waves in metals, Phys. Rev. B 77, 165135, 2008, https://doi.org/10.1103/PhysRevB.77.165135
2. J Low Temp Phys (2015) 178:355–366, DOI 10.1007/s10909-014-1253-y

Once the band structures at a dense k-grid is obtained by the FFT interpolation method, the above two properties of the Fermi surface can be calculated easily through the summation.

[Nacy820]

Hello, I have tried to calculate FS nesting function by EPW package, and the output is in the epw.out file. How can I extract these data and draw a figure? Is there a script you would like to share?