Coulomb truncation and convergence in a CNT
Posted: Mon Oct 17, 2016 4:06 pm
Dear yambo developers,
dear yambo community,
I often see that information on converging calculations of low-dimensional systems using yambo is scattered in this forum. As this meant a lot of trouble for me, I would like to share about my personal experience. This were strange convergence issues in low-dimensional systems in yambo 3.4.1 and 3.4.2 (latest release).
In this post, I would like to adress the convergence for the self-energy corrections in the GW scheme.
We converged the band gap of the (8,0)-CNT using a (1x1xN)-kgrid. The litarature value is 1.75 eV (Appl. Phys. A 78, 1129). As the system is quasi one-dimensional, Coulomb truncation has to be applied in order to converge the supercell. The random integration method (RIM) has to be activated in order to obtain meaningful results; the k-grid is (1x1x40). We note that we will never achieve supercell convergence:
If one does not activate RIM, the picture looks a bit strange:
The reason is that the GW self energy does not converge without RIM for an increasingly dense k-mesh and the resulting value of the gap has no meaning.
Now, we need to get rid of the convergence issue for the unit cell. We apply a Coulomb truncation using a cylinder of half the cell size. And the results that we find are somewhat discouraging:
[to be continued]
dear yambo community,
I often see that information on converging calculations of low-dimensional systems using yambo is scattered in this forum. As this meant a lot of trouble for me, I would like to share about my personal experience. This were strange convergence issues in low-dimensional systems in yambo 3.4.1 and 3.4.2 (latest release).
In this post, I would like to adress the convergence for the self-energy corrections in the GW scheme.
We converged the band gap of the (8,0)-CNT using a (1x1xN)-kgrid. The litarature value is 1.75 eV (Appl. Phys. A 78, 1129). As the system is quasi one-dimensional, Coulomb truncation has to be applied in order to converge the supercell. The random integration method (RIM) has to be activated in order to obtain meaningful results; the k-grid is (1x1x40). We note that we will never achieve supercell convergence:
If one does not activate RIM, the picture looks a bit strange:
The reason is that the GW self energy does not converge without RIM for an increasingly dense k-mesh and the resulting value of the gap has no meaning.
Now, we need to get rid of the convergence issue for the unit cell. We apply a Coulomb truncation using a cylinder of half the cell size. And the results that we find are somewhat discouraging:
[to be continued]