[Pw_forum] For help about Quantum Espresso

[Nicola Marzari]

Dear Liwei

1) BT is an insulator, so no smearing. 6 6 6 1 1 1 is an ultracareful
sampling for the cubic cell. use a much better convergence threshold
for the electrons - 10-8 or 10-10. if you double the supercell,
halve the kepoints in that direction - 6 6 3 1 1 1. Do not relax
lattice parameter and ions, and make sure you get exactly the same
values for the total energy.

2) you need to compare energies between the same
amounts of matter, and it can easily become a very complex
topic. try to read some extensive papers of people calculating the
stability of non-stoichiometric oxides.

3) usually pseudos generated in neutral configurations are good enough -
i.e. transferable enough.

nicola



On 3/21/11 5:19 PM, Liwei Geng wrote:
> Dear All,
>
> I am a newer to use Quantum Espresso. Could you please do me a favor to
> answer some little questions I met when using Quantum Espresso?
>
> 1. I am investigating the BaTiO3 crystal and used the best optimized
> parameter a=7.5344 Bohr. I used 'scf' to calculate the total energy.
> However, I obtained different values when using different number of
> cells. For example, when I use the unit cell, the total energy is -304
> Ry, but when I use 1X1X2 supercell, I obtained E=-598Ry, i.e., -299Ry
> per unit cell, when I increased to 2X2X4 supercell (the input file is
> below these questions), I got E=-4766Ry, i.e., -298 Ry per unit cell.
> What is wrong?
>
> 2. Another strange result is when I replaced two Ti atoms (Ti4+) by two
> Fe atoms (Fe3+) and removed one O atom (O2-) between them in the 2X2X4
> supercell, I obtained even lower energy than the pure BaTiO3 crystal.
> What is wrong? It isnot reasonable, right?
>
> 3. I use the atom pseudopotential file, but Ba4+ or Fe3+ are both ions,
> should I generate their ionic pseudopotential files?
>
> Following is my input file.
>
> &control
>
> calculation = 'scf',
>
> prefix = 'BaTiO3',
>
> pseudo_dir='/work/01553/geng'
>
> /
>
> &system
>
> degauss=0.01
>
> ibrav=6
>
> celldm(1)=15.0688
>
> celldm(3)=2
>
> nat=80
>
> ntyp=3
>
> ecutwfc=30.0
>
> ecutrho=300.
>
> /
>
> &electrons
>
> conv_thr = 1e-5,
>
> mixing_beta=0.7
>
> /
>
> &IONS
>
> ion_dynamics='bfgs',
>
> /
>
> &CELL
>
> cell_dynamics='damp-w',
>
> /
>
> ATOMIC_SPECIES
>
> Ba 137.327 Ba.pbe-nsp-van.UPF
>
> Ti 47.867 Ti.pbe-sp-van_ak.UPF
>
> O 15.9994 O.pbe-van_ak.UPF
>
> ATOMIC_POSITIONS
>
> Ba 0.000 0.000 0.000
>
> Ba 0.000 0.000 0.250
>
> Ba 0.000 0.000 0.500
>
> Ba 0.000 0.000 0.750
>
> Ba 0.500 0.000 0.000
>
> Ba 0.000 0.500 0.000
>
> Ba 0.500 0.500 0.000
>
> Ba 0.500 0.000 0.250
>
> Ba 0.500 0.000 0.500
>
> Ba 0.500 0.000 0.750
>
> Ba 0.000 0.500 0.250
>
> Ba 0.000 0.500 0.500
>
> Ba 0.000 0.500 0.750
>
> Ba 0.500 0.500 0.250
>
> Ba 0.500 0.500 0.500
>
> Ba 0.500 0.500 0.750
>
> Ti 0.250 0.250 0.125
>
> Ti 0.250 0.250 0.375
>
> Ti 0.250 0.250 0.625
>
> Ti 0.250 0.250 0.875
>
> Ti 0.750 0.250 0.125
>
> Ti 0.750 0.250 0.375
>
> Ti 0.750 0.250 0.625
>
> Ti 0.750 0.250 0.875
>
> Ti 0.250 0.750 0.125
>
> Ti 0.250 0.750 0.375
>
> Ti 0.250 0.750 0.625
>
> Ti 0.250 0.750 0.875
>
> Ti 0.750 0.750 0.125
>
> Ti 0.750 0.750 0.375
>
> Ti 0.750 0.750 0.625
>
> Ti 0.750 0.750 0.875
>
> O 0.250 0.000 0.125
>
> O 0.250 0.000 0.375
>
> O 0.250 0.000 0.625
>
> O 0.250 0.000 0.875
>
> O 0.750 0.000 0.125
>
> O 0.750 0.000 0.375
>
> O 0.750 0.000 0.625
>
> O 0.750 0.000 0.875
>
> O 0.000 0.250 0.125
>
> O 0.000 0.250 0.375
>
> O 0.000 0.250 0.625
>
> O 0.000 0.250 0.875
>
> O 0.000 0.750 0.125
>
> O 0.000 0.750 0.375
>
> O 0.000 0.750 0.625
>
> O 0.000 0.750 0.875
>
> O 0.250 0.500 0.125
>
> O 0.250 0.500 0.375
>
> O 0.250 0.500 0.625
>
> O 0.250 0.500 0.875
>
> O 0.750 0.500 0.125
>
> O 0.750 0.500 0.375
>
> O 0.750 0.500 0.625
>
> O 0.750 0.500 0.875
>
> O 0.500 0.250 0.125
>
> O 0.500 0.250 0.375
>
> O 0.500 0.250 0.625
>
> O 0.500 0.250 0.875
>
> O 0.500 0.750 0.125
>
> O 0.500 0.750 0.375
>
> O 0.500 0.750 0.625
>
> O 0.500 0.750 0.875
>
> O 0.250 0.250 0.000
>
> O 0.250 0.250 0.250
>
> O 0.250 0.250 0.500
>
> O 0.250 0.250 0.750
>
> O 0.750 0.250 0.000
>
> O 0.750 0.250 0.250
>
> O 0.750 0.250 0.500
>
> O 0.750 0.250 0.750
>
> O 0.250 0.750 0.000
>
> O 0.250 0.750 0.250
>
> O 0.250 0.750 0.500
>
> O 0.250 0.750 0.750
>
> O 0.750 0.750 0.000
>
> O 0.750 0.750 0.250
>
> O 0.750 0.750 0.500
>
> O 0.750 0.750 0.750
>
> 　
>
> K_POINTS {automatic}
>
> 20 20 10 0 0 0
>
> Thank you very much!
>
> Best regards,
>
> Liwei
>
>


-- 
----------------------------------------------------------------------
Prof Nicola Marzari    Department of Materials    University of Oxford
Chair of Materials Modelling  Director, Materials Modelling Laboratory
nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM