Bulk material: h-BN

In this tutorial you will learn how to generate the Yambo SAVE folder for bulk hBN starting from a PWscf calculation.

Prerequisites

You will need:

• PWSCF input files and pseudopotentials for hBN bulk (Download here)
• pw.x executable, version 5.0 or later
• p2y executable

System characteristics

Atomic structure of bulk hBN

Hexagonal boron nitride - hBN:

• HCP lattice, ABAB stacking
• Four atoms per cell, B and N (16 electrons)
• Lattice constants: a = 4.716 [a.u.], c/a = 2.582
• Plane wave cutoff 40 Ry (1500 RL vectors in wavefunctions)
• SCF run: shifted 6x6x6 grid with 8 bands
• Non-SCF run: unshifted 6x6x2 grid with 100 bands

DFT calculations

Unpack the tarfile. It uses the same file structure as other yambo tutorials:

$ tar -xcvf hBN-bulk.tar
$ cd YAMBO_TUTORIALS/hBN/PWSCF
$ ls
Inputs		Pseudos		PostProcessing		References
hBN_scf.in	hBN_nscf.in     hBN_scf_plot_bands.in  hBN_nscf_plot_bands.in 

First run the SCF calculation to generate the ground-state charge density, occupations, Fermi level, and so on:

pw.x < hBN_scf.in > hBN_scf.out

The output reports 36 k-points. The valence band maximum is at 5.13eV.

Next run a non-SCF calculation to generate a set of Kohn-Sham eigenvalues and eigenvectors across a denser k-point mesh, for occupied and unoccupied states:

pw.x < hBN_nscf.in > hBN_nscf.out

Note the presence of the following flags in the input file:

wf_collect=.true.
force_symmorphic=.true.

which are needed for generating the Yambo databases. Full explanations of these variables are given on the quantum-ESPRESSO input variables page.

After these two runs, you should have a hBN.save directory:

$ ls hBN.save
data-file.xml charge-density.dat gvectors.dat B.pz-vbc.UPF N.pz-vbc.UPF
K00001	K00002 .... 	K00035	K00036

Conversion to Yambo format

The PWscf bBN.save output is converted to the Yambo format using the p2y executable (pwscf to yambo), found in the yambo bin directory. Enter hBN.save and launch p2y:

$ cd hBN.save
$ p2y
...
<---> DBs path set to .
<---> Index file set to data-file.xml
<---> Header/K-points/Energies... done
...
<---> == DB1 (Gvecs and more) ...
<---> ... Database done
<---> == DB2 (wavefunctions)  ... done ==
<---> == DB3 (PseudoPotential) ... done ==
<--->  == P2Y completed ==

This output repeats some information about the system and generates a SAVE directory:

$ ls SAVE
ns.db1  ns.wf  ns.kb_pp_pwscf
ns.wf_fragments_1_1 ...
ns.kb_pp_pwscf_fragment_1 ...

These files, with an n prefix, indicate that they are in netCDF format, and thus not human readable. However, they are perfectly transferable across different architectures.

You are now ready to run Yambo. Check that the databases contain the information you expect:

$ yambo -D
[RD./SAVE//ns.db1]------------------------------------------
Bands                           : 100
K-points                        : 14
G-vectors             [RL space]:  8029
Components       [wavefunctions]: 1016
...
[RD./SAVE//ns.wf]-------------------------------------------
Fragmentation                    :yes
...
[RD./SAVE//ns.kb_pp_pwscf]----------------------------------
Fragmentation                    :yes
- S/N 006626 -------------------------- v.04.01.02 r.00000 -

In practice we suggest to move the SAVE into a new clean folder. In this tutorial however, we ask instead that you continue using a SAVE we prepared previously:

$ cd ../../YAMBO
$ ls
SAVE

Advanced users

p2y accepts several command line options:

$ p2y -H
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