[Pw_forum] For help about Quantum Espresso

[Liwei Geng]

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 


 
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