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25

These are a few extra points to complement Andrew Rosen's comprehensive response: To be absolutely clear, typical DFT calculations are not performed at 0K, a better description of what happens is that they are performed "for a static crystal". Static crystal means that the atoms are fixed at their crystallographic positions (which is what a typical DFT ...


20

You are correct that KS-DFT, strictly speaking, involves calculations of a potential energy surface at 0 K. However, if you accept that the density functional approximation you are using is sufficiently accurate, it is not too difficult of a stretch to go from 0 K to finite temperature conditions for an application of interest. The key assumption is that the ...


11

$\Delta$SCF This method generates excited states by changing the occupancy of a ground state determinant and then carrying out a new SCF with that initial guess, with some restriction throughout to prevent variational collapse back to the ground state [1]. The most common approach to stay out of the ground state is the Maximum Overlap Method (MOM), which ...


11

Kohn-Sham DFT may only be rigorous at zero temperature, but at nonzero temperature, Kohn-Sham-Mermin DFT is an equally rigorous replacement. There are two major differences Rather than deriving the orbital equations from a minimization of the energy, $E$, one minimizes the free energy $F = E - TS$, where $S$ is the entropy. A practical consequence is ...


9

GW+BSE: Excited states in the framework of many-body Green's function comprise charged excitations, where the number of electrons in the system changes from $N$ to $N-1$ or $N + 1$, and natural excitations, where the number of electrons remains constant. In the $|N\rangle \rightarrow |N-1\rangle$ case, an electron in the valence band (occupied orbital) is ...


8

I am going to try to go systematically through your question and answer the critical bits. I may edit this answer going forward as I'm able to express my thoughts better or people point out important subtleties that I may be missing. First, in KS-DFT (without any Generalized KS (GKS) or w/e), $V_{xc}$ is explicitly multiplicative and local. So it is ...


8

A lot of it will be in the difference in cost. But as a rule of thumb, GW potentials are typically harder, and include more states (semicores). This need not always be the case, since if you look at the Ag POTCARS, there isn't any difference at all in the recommended ENMIN/ENMAX, partial core radius, or even the states that are considered valence. In ...


8

The GPAW code can do GW/BSE and it uses purely PAW potentials. So it's absolutely possible to do GW/BSE with PAW sets. The involvement for the implementation, however, is another topic. From Kevin J. M.'s answer, the primary block is fairly clear: PAWs are quite involved to implement. Doing all of the atom-centered corrections and then getting them onto your ...


8

I am not aware of any public codes that have a force implementation for excited state calculations using the Bethe-Salpeter equation (happy to be corrected on this front). However, the methodology to do this was published some time ago by Ismail-Beige and Louie in this paper, where they also have an in-house implementation that they use to validate the ...


8

The term absorbance does not have anything directly related to the GW approximation or the Bethe-Salpeter equation (BSE). The "absorbance" is a property of a material, while GW and BSE are just methods of calculating things. It's similar to saying that the conductivity of a material has to do with how much a material allows the flow of electrons, ...


6

Not a direct answer to your proposed approximation to the exciton group velocity as $\mathbf{Q}\to0$, but just wanted to point out that the latest version of Yambo supports the calculation of exciton dispersions, from which you can then calculate the exciton velocity at any $\mathbf{Q}$. Here are the details: Yambo 5: exciton dispersion.


4

Here I show the typical calculation flow of GW in VASP: a DFT ground-state calculation. obtain DFT virtual orbitals GW calculation including LWANNIER90 TAG Compute Wannier functions and Obtain bandstructure by Wannier interpolation. PS1: use POTCAR like [XXX_GW]. PS2: If you want to include spin-orbit coupling, just add the following tags: LSORBIT=.TRUE....


3

The computational chemistry software ORCA (requires the registration of an account, although both registration and the software itself is free for academic users; for a tutorial that is viewable without registration, see https://www.orcasoftware.de/tutorials_orca/index.html). In ORCA one can compute XAS/XES/RIXS at the TDDFT, DFT/ROCIS (a spin-adapted, ...


3

Your question is not so focus. I assume that you want to read the BSE oscillator strength from the VASP output. I strongly advise you that take a look at this link: https://www.vasp.at/wiki/index.php/Plotting_the_BSE_fatband_structure_of_Si After the (model)BSE calculation we first look into the [vasprun.xml] file to have a look at the BSE eigenvalues and ...


2

Found a small script for BGW to do the plot https://github.com/BerkeleyGW/bgwtools/blob/master/bgwtools/BSE/plot_envelope.py.


2

I think these figures are plotting the square of $A_{vc\vec{k}}$, therefore no phase information is contained in these figures. Note that the figures that you are showing are marking each excitonic state with $s$, $p$, $d$, and $f$, which represent exactly the square of the wave function in the problem of the hydrogen atom. The exciton character and ...


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