[Pw_forum] FW: cell optimization results seems to be a little strange

[vega lew]

Dear all,

Thank to all your help, I could run a Q-E calculation on my cluster more correctly. Now I can learn to do some 
simple calculations.

My first job is to calculate the Anatase-TiO2 lattice cell parameter using Q-E and compare it with literature.
I have tested the BFGS for vc-relax, but the cell parameters changed so much that an error was occured with
the information 'non orthogonal operation'. Then I used a symmetry-conserving algorithm as the QE manual
said. The input file as following,
 
&CONTROL
                       title = 'Anatase lattice' ,
                 calculation = 'vc-relax' ,
                restart_mode = 'from_scratch' ,
                      outdir = '/home/vega/espresso-4.0/tmp/' ,
                      wfcdir = '/tmp/' ,
                  pseudo_dir = '/home/vega/espresso-4.0/pseudo/' ,
                      prefix = 'Anatase lattice default' ,
                     disk_io = 'none' ,
               etot_conv_thr = 0.000000735 ,
               forc_conv_thr = 0.0011668141375 ,
                       nstep = 1000 ,
 /
 &SYSTEM
                       ibrav = 6,
                   celldm(1) = 7.135605333,
                   celldm(3) = 2.5121822033898305084745762711864,
                         nat = 12,
                        ntyp = 2,
                     ecutwfc = 25 ,
                     ecutrho = 200 ,
 /
 &ELECTRONS
                    conv_thr = 7.3D-8 ,
 /
 &IONS
                ion_dynamics = 'damp' ,
 /
 &CELL
               cell_dynamics = 'damp-w' ,
 /
ATOMIC_SPECIES
   Ti   47.86700  Ti.pw91-sp-van_ak.UPF 
    O   15.99940  O.pw91-van_ak.UPF 
ATOMIC_POSITIONS angstrom 
   Ti      0.000000000    0.000000000    0.000000000    
   Ti      1.888000000    1.888000000    4.743000000    
   Ti      0.000000000    1.888000000    2.372000000    
   Ti      1.888000000    0.000000000    7.115000000    
    O      0.000000000    0.000000000    1.973000000    
    O      1.888000000    1.888000000    6.716000000    
    O      0.000000000    1.888000000    4.345000000    
    O      1.888000000    0.000000000    9.088000000    
    O      1.888000000    0.000000000    5.141000000    
    O      0.000000000    1.888000000    0.398000000    
    O      1.888000000    1.888000000    2.770000000    
    O      0.000000000    0.000000000    7.513000000    
K_POINTS automatic
 7 7 3   1 1 1 

The calculation can be done nearly 600 fs. The last few lines of results file is shown at the bottom.
But the cell parameters for Anatase lattice seems to be not perfectly match the results in literature.
In the literature, Journal of Physical Chemistry B, 2006, 110, 7464, the author used  a conjugate-gradient 
or quasi-Newton scheme as implemented in VASP to optimaize the geometries. And the cell parameters
are a = b = 3.803 A c = 9.603 A c/a = 2.525. But my Q-E result, a= 3.7546166A b= 3.75461729A c= 9.3911A
seems to be much smaller than his. I tried times times again, even, the starting geometry is from the 
exprimental data for Journal of  the American Society, 1987,109, 3639, which was also cited by him, the
result was still that small. 

I also have tested it in CASTEP, using BFGS and same precision. The CASTEP results were a = b =3.798668A
c = 9.693431A c/a = 2.5518, which seemed to be more close to the literature results. The starting geometry
was also from Journal of  the American Society, 1987,109, 3639. 

I think maybe some parameter in my Q-E input file was wrong. Could you please point it to me?

thank you for reading. I am waiting for your responding.

Vega Lew
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


_________________________________________________________________
Connect to the next generation of MSN Messenger 
http://imagine-msn.com/messenger/launch80/default.aspx?locale=en-us&source=wlmailtagline
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.democritos.it/pipermail/pw_forum/attachments/20080703/b6db6211/attachment.htm