[Pw_forum] something strange in rutile and anatase calculation

[vega]

Dear all,

It is known that the rutile is more stable than anatase in thermodynamics. Surprisingly, when I did the vc-relax calculation by QE, I found the calculated energy of anatase lower than rutile. Both vc-relax calculation for rutile and anatase onverged in 6 scf cycles and 3 bfgs steps, giving the final enthalpy = -362.7585836890 Ry for rutile and -725.5447425835 Ry for antase. So the average energy of [TiO2] unit is -181.3792918445 Ry for rutile and -181.386185645875 Ry for anatase. It looks like anatase is more stable than rutile. Do you think it was quite strange?

To make the calculation both for anatase and rutile comparable, I used the same ecutwfc=40 Ry and ecutwfc=400 Ry, and a quite dense k-point mesh for both case, 4x4x7 for rutile and 5x5x2 for anatase.

for better understanding in my calculation, please read the input file below,
for anatase case,
&CONTROL
                       title = 'Anatase lattice' ,
                 calculation = 'vc-relax' ,
                restart_mode = 'from_scratch' ,
                      outdir = '/home/hjfeng/vega/TiO2/Anatase/lattice/vctest/tmp/' ,
                      wfcdir = '/tmp/' ,
                  pseudo_dir = '/home/hjfeng/vega/espresso-4.0/pseudo/' ,
                      prefix = 'Anatase lattice' ,
                     disk_io = 'none' ,
                       nstep = 1000 ,
 /
 &SYSTEM
                       ibrav = 6,
                   celldm(1) = 7.1356,
                   celldm(3) = 2.5122,
                         nat = 12,
                        ntyp = 2,
                     ecutwfc = 40 ,
                     ecutrho = 400 ,
 /
 &ELECTRONS

 /
 &IONS
                ion_dynamics = 'bfgs' ,
 /
 &CELL
               cell_dynamics = 'bfgs' ,
 /
ATOMIC_SPECIES
   Ti   47.86700  Ti.pw91-sp-van_ak.UPF 
    O   15.99940  O.pw91-van_ak.UPF 
ATOMIC_POSITIONS crystal 
   Ti      0.000000000    0.000000000    0.000000000    
   Ti      0.500000000    0.500000000    0.500000000    
   Ti      0.000000000    0.500000000    0.250000000    
   Ti      0.500000000    0.000000000    0.750000000    
    O      0.000000000    0.500000000    0.042000000    
    O      0.000000000    0.000000000    0.208000000    
    O      0.500000000    0.500000000    0.292000000    
    O      0.000000000    0.500000000    0.458000000    
    O      0.500000000    0.000000000    0.542000000    
    O      0.500000000    0.500000000    0.708000000    
    O      0.000000000    0.000000000    0.792000000    
    O      0.500000000    1.000000000    0.958000000    
K_POINTS automatic 
  5 5 2   1 1 1 

for rutile case,

    &CONTROL
                       title = Rutile_lattice ,
                 calculation = 'vc-relax' ,
                restart_mode = 'from_scratch' ,
                      outdir = '/home/vega32/TiO2/rutile/lattice/vcrelax/tmp' ,
                      wfcdir = '/tmp/' ,
                  pseudo_dir = '/home/vega32/espresso-4.0.1/pseudo/' ,
                      prefix = 'Anatase lattice' ,
                     disk_io = 'none' ,
                       nstep = 1000 ,
 /
 &SYSTEM
                       ibrav = 6,
                   celldm(1) = 8.6814,
                   celldm(3) = 0.6441,
                         nat = 6,
                        ntyp = 2,
                     ecutwfc = 40 ,
                     ecutrho = 400 ,
 /
 &ELECTRONS

 /
 &IONS
                ion_dynamics = 'bfgs' ,
 /
 &CELL
               cell_dynamics = 'bfgs' ,
 /
ATOMIC_SPECIES
    O   15.99940  O.pw91-van_ak.UPF
   Ti   47.90000  Ti.pw91-sp-van_ak.UPF
ATOMIC_POSITIONS crystal    
    O      0.304800000    0.304800000    0.000000000    
    O      0.695200000    0.695200000    0.000000000    
    O      0.195200000    0.804800000    0.500000000    
    O      0.804800000    0.195200000    0.500000000    
   Ti      0.500000000    0.500000000    0.500000000    
   Ti      0.000000000    0.000000000    0.000000000    
K_POINTS automatic 
  4 4 7   1 1 1 

vega

==============================================================
Vega Lew ( weijia liu)
PH.D Candidate in Chemical Engineering
State Key Laboratory of Materials-oriented Chemical Engineering
College of Chemistry and Chemical Engineering
Nanjing University of Technology, 210009, Nanjing, Jiangsu, China
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