## Fiesta: a Gaussian-basis GW and Bethe-Salpeter code

This is a very minimal Fiesta website. Namely, we are in construction ... Detailed features, benchmark calculations, examples, are "soon" to come, even though all this can be found in the related papers (see Publications list).

** Fiesta ad minima. **
The Fiesta code implements the GW and Bethe-Salpeter formalisms using Gaussian bases and
resolution-of-the-identity techniques (RI-SVS density and RI-V Coulomb metric). Dynamical
screening contribution to the self-energy is explicitely accounted for through a contour
deformation approach. Self-consistency on the wavefunctions is implemented at the static
COHSEX level. Tamm-Dancoff approximation (TDA) or full Bethe-Salpeter
calculations can be performed. The code presently reads input Kohn-Sham eigenstates
from the open-source Siesta and
NWChem packages so that
all-electron or pseudopotential calculations
can be performed with standard quantum chemistry bases or with the numerical orbitals
generated by the Siesta package (requesting then a Gaussian fit of the radial part of the basis).
Any DFT code dumping all Kohn-Sham eigenstates (occupied/unoccupied) expressed on a Gaussian basis,
plus the exchange-correlation contribution to the Kohn-Sham eigenvalues, can be branched very
straighforwardly onto the Fiesta code.

The Fiesta code now implements ** continuous polarizable models (PCM) ** and is merged with
the Mescal ** Discrete Polarizable Model (DPM) ** [see: D'Avino et al., JCTC 2014;
Li, D'Avino et al. JPC Lett. 2016] so as to provide embedded GW and Bethe-Salpeter QM/MM formalisms.

The Fiesta code is evolving towards a cubic scaling "space-time" approach following the methodology
implemented at the RPA level (see Duchemin, Blase, arXiv:1902.00066).

** Not started yet: ** periodic boundary conditions. As it stands, Fiesta allows only finite
size systems (clusters, molecules) calculations.

** Fiesta is not yet an open-source package freely distributed, even though it is shared by
a few academic friendly partners. **

The Fiesta code is written in F90/MPI. It was initially developed having in mind to run GW/BSE
calculations for moderate size systems (~100 atoms) on a standard desk PC (e.g. 8 cores/32 Go)
with a reasonable basis (e.g. 6-311G**). If you happen to have access to larger computers, the code
has been shown to sustain excellent scaling up to several thousand cores, allowing GW/BSE
calculations on several hundred atoms with e.g. aug-cc-pvtz quality basis.
**Left image:** Performance of one GW iteration for one or two fullerenes (RI-SVS metric,
DZP basis). The processor grid size varies from 16 to 61440 cores. 138 TeraFlops were reached
for the largest grid (European PRACE project, Ivan Duchemin, L_sim/INAC/CEA/Grenoble).