[Pw_forum] Serious problem in total energy calculation ?

["이은철"]

Dear advanced users and developers of PWSCF;

I recently installed Quantum Espresso code, ver 3.2.2, and 
performed simple calculations for tests using PWSCF. 
Today, I have found that total energy of a single oxygen atom
calculated in PWSCF is different from that generated in  Vanderbilt's 
pseudopotential code (ver. 7.3.6). I surely remember that the total
energies of single atom  in pseudopotential generation program
and in plane wave code are almost same within small errors, when
I used Berkely code made by S. Froyen. Please inform me if I make
any mistake in using PWSCF code, after seeing following results.

1. Total energy of a single oxygen atom calculated by Vanderbilt's 
pseudopotential code;

You can easily find it in the header of the oxygen pseudopotential file
O.pbe-van_bm.UPF at PWSCF webpage, as follows. 

<PP_HEADER>
   0                   Version Number
   O                   Element
   US                  Ultrasoft pseudopotential
    F                  Nonlinear Core Correction
 SLA  PW   PBE  PBE    PBE  Exchange-Correlation functional
    6.00000000000      Z valence
  -31.58339463984      Total energy
 25.000000 100.0000000 Suggested cutoff for wfc and rho
    1                  Max angular momentum component
  737                  Number of points in mesh

Thus, this file indicate that the total energy of single oxygen atom is 
-31.58339463984 Ry. 

2. PWSCF calculation
     
I used following input file. 

&control
    calculation='relax',
    restart_mode='from_scratch',
    prefix='o',
    pseudo_dir = '/home/kgna0022/pseudo/',
    outdir='/scratch2/kgna0022/tmp/',
    nstep=1,
 /
 &system
    ibrav= 1,
    celldm(1)=20.0
    nat= 1, ntyp= 1,
    ecutwfc =30.0,
    ecutrho =210.0,
    nbnd=6,
    occupations='from_input',
 /
 &electrons
    conv_thr =  1.0D-8,
    mixing_beta = 0.7D0,
/
&IONS
  pot_extrapolation = "second_order",
  wfc_extrapolation = "second_order",
/
ATOMIC_SPECIES
 O   16.00  O.pbe-van_bm.UPF
ATOMIC_POSITIONS (bohr)
O        0.00000000000   0.000000000   0.000000000
K_POINTS (Gamma)
OCCUPATIONS
2.0  1.333333333333333    1.333333333333333   1.333333333333333  0.0 0.0

After running PWSCF, I obtained following results.

     total energy              =   -31.53644545 Ry
     total energy              =   -31.53655807 Ry
     total energy              =   -31.53657629 Ry
!    total energy              =   -31.53657705 Ry

Thus, the total energy is -31.53657705 Ry, diffent from
the result of Vanderbilt's code, -31.58339463984 Ry. 

If you teach me my mistake during the calculations, I will
be happy. However, if you don't find my mistake, I cordially ask 
developers of PWSCF code to solve this problem.
Thank you very much.

Best regards,

Eun-Cheol Lee, Ph. D.
Assistant Professor,
Dept. of BioNano Tech., 
Kyungwon University, Korea.
E-mail : eclee at kyungwon.ac.kr
TEL : +82-31-750-8752
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