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
- 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
- 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.
The Fermi surface nesting function$\xi_\mathbf{q}$ is defined as:
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:
where$f$ is the Fermi-Dirac function.
These defintions can be referred to
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.