# 2. Getting Started¶

This section is intended to give a first impression on how a typical calculation is carried out. Details are given in later sections.

Many-body perturbation theory is defined with respect to a formally unperturbed system, i.e., a system without electron-electron interaction. Any mean-field theory, e.g., Hartree-Fock or Kohn-Sham (KS) DFT, provides such a system. (For simplicity of presentation, we will assume throughout this manual that KS-DFT is used for the reference system.) Thus, before running Spex, we must find a self-consistent mean-field solution. Normally this first step is fast and we can use a dense k-point set. For the calculation with Spex, we have to generate a new (and usually coarser) k-point set and calculate the corresponding wavefunctions and energies with the mean-field program in a second run. Thus, a typical procedure is

- self-consistent DFT calculation (e.g., with Fleur),
- generate new k-point set with Spex,
- second run of the DFT program to generate the eigenstates at the new k points,
- finally run Spex.

Of course, for subsequent Spex runs, steps 1-3 need not be repeated, unless the reference mean-field system is to be
recalculated with different parameters (e.g., different k-point set or different crystal structure).
For the first and third step consult the manual of your DFT program. (The Fleur manual is available online. As explained in more detail in
Section 8, you must set `gw=1`

and `gw=2`

for the first and third step, respectively.)
For the second and fourth step you need to create an input file “spex.inp” for Spex. A very simple input file for a *GW* calculation of Si is this:

```
# Quasiparticle energies of bands at the high-symmetry points Gamma, X, and L.
BZ 4 4 4
WRTKPT
KPT X=[0,0,1] L=1/2*[1,1,1]
JOB GW 1:(1,2,5) X:(1,3,5) L:(1-3,5)
```

Note

Refer to the tutorials and other chapters for a full list of available keywords

For the second step, only the lines beginning with `BZ`

and `WRTKPT`

are necessary. The line `BZ 4 4 4`

defines a \(4\times 4\times 4\) k-point set, and the keyword `WRTKPT`

tells Spex to write the (irreducible) k-points to a file (“kpts” for Fleur) and stop.

For the fourth step, you must remove (or comment out) the keyword `WRTKPT`

. Now all other parameters are needed.
The results are written to standard output and should be piped to an output file (Section 3.3).

Note

As an alternative to `WRTKPT`

, the command line option `-w`

can be used (`spex -w`

). It sets the keyword `WRTKPT`

automatically (Section 3.1).

Note

Unless you intend to run self-consistent GW calculations, the steps 2-4 can be combined into a single Spex run using the keyword `ITERATE`

.