How to choose the input parameters
In the previous tutorial, you have been guided step-by-step through the calculations of the optical spectrum of bulk hBN by solving the Bethe-Salpeter equation. The values for the relevant input parameters have then been given to you.
In this tutorial you will learn how to choose those parameters. These parameters are related either to the truncation of infinite sums or to the approximations of infinitesimal with small, but finite quantities. A wrong choice of these parameters can lead to inaccurate or even physically wrong results. One then needs to run a series of calculations by changing the parameters till the results are converged , meaning they are changing by a negligible amount.
Note that all the following operations can be automated. A useful python-based interface to yambo, yambopy, can be used for this purpose. It is worth however to go at least once through the pain of a 'by-hand' convergence study so to better understand the automated process for few of those variables.
Coming back to the scheme to calculate the macroscopic dielectric matrix within Bethe-Salpeter, here the list of the parameters that have to be determined by convergence studies:
Though we will discuss those parameters, we will run examples only for the BS kernel part.
Convergence of the static screening
The microscopic dielectric function is related to X by:
Convergence of the macroscopic dielectric function
where vck indicates the pair of quasiparticle states vk and ck. The first term on the RHS is the quasiparticle energy differences (diagonal only). The second term is the kernel which is the sum of the electron-hole exchange part V (which stems from the Hartree potential) and the electron-hole attraction part W (which stems from the screened exchange potential). The kernel both shifts (diagonal contributions) and couples (off-diagonal contributions) the quasiparticle energy differences:
