FAQ#
Last update: 2026-08-03
Features and Functionalities#
Can you recommend some reading material on the background of the NEGF method?#
For first reading, there are several good introductions published by Supriyo Datta.
The book is an introduction to NEGF which is not too advanced.
There is a more advanced book, [DattaETMS1995]. Chapter 8 is a good introduction.
A rather simple article is [DattaSuperlatticesMicrostructures2000].
[Jirauschek2014] describes the NEGF method in Section VIII.
Other books or articles on NEGF usually require advanced quantum mechanics or quantum field theory.
What is the difference between the “RealSpaceModes” and “EnergyEigenstates” in the output folder?#
“EnergyEigenstates” refers to the energy eigenstates of given heterostructure, obtained by diagonalizing the Hamiltonians (at each in-plane k point). “RealSpaceModes” are the position eigenstates within the subspace obtained after applying the axial cut-off energy (see Reduced real space basis). These position eigenstates are used as a basis in the NEGF calculation.
Note
“RealSpaceModes” depends on Axial energy cut-off. Increasing the axial energy range localizes the position eigenstates more.
What are the meanings of “Check normalization of spectral function = …” and “Check normalization of lesser Green’s function = …” in the log file?#
If the final values differ significantly from 1, these are indications that some states are not correctly accounted by the energy grid: either EnergyGridSpacing is too coarse, and/or the energy range is not correctly defined. See Energy range of the Green’s functions for how to resolve the latter issue.
Error Messages#
nextnano.NEGF exit code: -…#
Simulation *.log file contains exit code with a big negative number
(nextnano.NEGF exit code: ...)
Most likely you need to install the Microsoft Visual C++ Redistributable . Choose the corresponding version matching your operation system architecture (most likely X64) from the section Latest Microsoft Visual C++ Redistributable Version.
Problems and Limitations#
How much memory do I need?#
We recommend 16 GB or larger RAM. 8 GB may be sufficient for some input files, but the calculation becomes incredibly slow if there is not sufficient memory available.
You should run heavier simulations on a computer having 32 GB of RAM. 16 GB RAM might require you to reduce the number of parallel threads used for the gain calculation. We recommend a Windows PC with 64 GB RAM and recent CPU, or a Linux machine with comparable performance.
My results do not agree with experiment.#
A discrepancy in the used effective masses could be an explanation. In addition, the Interface roughness scattering plays an important role. The values given in the paper of A. Wacker seem to be taken in order to fit the experimental data. However, their model for elastic scattering is simplified (with no in-plane dependence for the scattering processes), so it might be that the actual interface roughness is different in reality.
Why does the current density vary at different places? Current does not seem to be conserved.#
The NEGF calculation is done within the mode space basis that depends on Axial energy cut-off. Mathematically speaking, the Hilbert space of the Green’s functions has lower dimension than the initial Hamiltonian which had the full real space resolution (SpatialGridSpacing). As a consequence, the current calculated from the solution of the Dyson-Keldysh equations is not exactly conserved in real space, especially through high barriers. The current should be better conserved when more subbands are selected.
In the gain spectra, the energy grid is not as expected from the input dEPhot/dEPhotSelfConsistent.#
The energy interval for the gain calculation will be set to at least the EnergyGridSpacing.