# SIESTA wrapper

SIESTA is a localized-orbital DFT code, available at: https://departments.icmab.es/leem/siesta/ It can be used in conjunction with BerkeleyGW, especially to generate unoccupied states with the SAPO utility.

Warning

Make sure you understand when you can use localized basis sets for GW calculations. When if doubt, use a more well-tested and validated wrapper based on Quantum ESPRESSO, PARATEC, or Abinit.

### siesta2bgw.x

An example siesta2bgw.x calculation is provided in examples/DFT/benzene. Please see this example for usage and example siesta and denchar input files.

The SIESTA wrapper reads lattice vectors, atomic species and positions from SystemLabel.XV file, k-points from SystemLabel.KP file, eigenvalues from SystemLabel.EIG file, wavefunctions in R-space from SystemLabel{.K$k}.WF$n{.UP|.DOWN}{.REAL|.IMAG}.cube file, symmetry group, kinetic energy cutoffs for wavefunctions and charge density, k-grid dimensions and offsets from the input file, generates reciprocal lattice vectors, rotation matrices, fractional translations, FFT grid and G-vectors, performs FFT from R-space to G-space, and writes the selected range of bands to the output WFN file. Make sure to set up the real space grid in SIESTA/DENCHAR utility same as FFT grid. If the two grids differ the SIESTA wrapper will return an error. The output WFN file can be used as an auxiliary wavefunction for the SAPO code.

A full GW/BSE calculation with SIESTA wavefunctions also requires RHO and VXC files. These files can be generated from SystemLabel.RHO{.UP|.DN}.cube, SystemLabel.VT{.UP|.DN}.cube and SystemLabel.VH.cube files obtained with SIESTA/GRID2CUBE utility on SIESTA real space grid. Make sure SIESTA real space grid is equivalent to FFT grid by setting parameter MeshCutoff in SIESTA input file same as kinetic energy cutoff for charge density.

Input is read from file siesta2bgw.inp:

&input_siesta2bgw
systemlabel      = 'ammonia'
wfng_output_file = 'wfng.lo'
rhog_output_file = ''
vxcg_output_file = ''
ecutwfn          = 60.0
ecutrho          = 240.0
wfng_nk1         = 1
wfng_nk2         = 1
wfng_nk3         = 1
wfng_dk1         = 0.0
wfng_dk2         = 0.0
wfng_dk3         = 0.0
wfng_ref_flag    = .true.
wfng_ref_kpoint  = 1
wfng_ref_spin    = 1
wfng_ref_band    = 4
wfng_ref_energy  = -5.87
wfng_band_flag   = .true.
wfng_band_min    = 5
wfng_band_max    = 28
wfng_energy_flag = .false.
wfng_energy_min  = 0.0
wfng_energy_max  = 0.0
wfng_gamma_real  = .true.
/


Here, lattice vectors, k-points, eigenvalues and wavefunctions in R-space are read from ammonia.XV, ammonia.KP, ammonia.EIG and ammonia.WF\$n.cube files, respectively. The kinetic-energy cutoffs for wavefunction and charge density are set to 60 and 240 Ry, respectively. The k-grid is set to the Gamma-point. The SIESTA eigenvalues are shifted in energy so that the HOMO state (1st k-point, 1st spin, 4th band) appears at -5.87 eV. The real parts of resonant SIESTA wavefunctions (from 5th to 28th band) are read from Gaussian Cube files and the imaginary parts are set to zero (wfng_gamma_real), the wavefunctions are brought from R-space to G-space and written to wfng.lo file. The SIESTA charge density and exchange-correlation potential files are not generated.