[Pw_forum] relaxation of even-layer slab
lucky8121
lucky8121 at sohu.com
Thu Oct 12 04:38:52 CEST 2006
Dear Prof. Baroni，
Thanks for your prompt reply.
(1) The four-layer slab
I did get warnings about the setup of the four-layer slab in the output file:
warning: symmetry operation # 4 not allowed. fractional translation:
0.3333333 -0.3333333 0.0000000 in crystal coordinates
warning: symmetry operation # 11 not allowed. fractional translation:
0.3333333 -0.3333333 0.0000000 in crystal coordinates
warning: symmetry operation # 12 not allowed. fractional translation:
0.3333333 -0.3333333 0.0000000 in crystal coordinates
warning: symmetry operation # 13 not allowed. fractional translation:
0.3333333 -0.3333333 0.0000000 in crystal coordinates
warning: symmetry operation # 19 not allowed. fractional translation:
0.3333333 -0.3333333 0.0000000 in crystal coordinates
warning: symmetry operation # 20 not allowed. fractional translation:
0.3333333 -0.3333333 0.0000000 in crystal coordinates
The real-space FFT grids in this calculation were:
G cutoff = 146.8225 ( 47267 G-vectors) FFT grid: ( 25, 25,180)
G cutoff = 73.4112 ( 16847 G-vectors) smooth grid: ( 18, 18,125)
(2) The six-layer slab
Then I performed a calculation of a six-layer slab. The initial positions were:
Cu 0.5000000 0.2886751 -1.632993
Cu 0.0000000 0.5773505 -0.816497
Cu 0.0000000 0.0000000 0.000000
Cu 0.5000000 0.2886751 0.816497
Cu 0.0000000 0.5773503 1.632993
Cu 0.0000000 0.0000000 2.449490
After relaxation, the final positions are:
Cu 0.500000000 0.288675100 -1.626940097
Cu 0.000000000 0.577350500 -0.819973440
Cu 0.000000000 0.000000000 -0.003573650
Cu 0.500000000 0.288675100 0.818345337
Cu 0.000000000 0.577350300 1.634784508
Cu 0.000000000 0.000000000 2.446847342
The interlayer spacings on the two sides of the slab are 0.812062834 and 0.806966657, respectively. This time the asymmetry becomes larger than the case of four-layer slab.
I noticed that in the output file there are warnings as below:
warning: symmetry operation # 4 not allowed. fractional translation:
0.3333334 -0.3333333 -0.0909088 in crystal coordinates
warning: symmetry operation # 11 not allowed. fractional translation:
0.3333334 -0.3333333 -0.0909088 in crystal coordinates
warning: symmetry operation # 12 not allowed. fractional translation:
0.3333334 -0.3333333 -0.0909088 in crystal coordinates
warning: symmetry operation # 13 not allowed. fractional translation:
0.3333334 -0.3333333 -0.0909088 in crystal coordinates
warning: symmetry operation # 19 not allowed. fractional translation:
0.3333334 -0.3333333 -0.0909088 in crystal coordinates
warning: symmetry operation # 20 not allowed. fractional translation:
0.3333334 -0.3333333 -0.0909088 in crystal coordinates
And the FFT grids are:
G cutoff = 146.8225 ( 57757 G-vectors) FFT grid: ( 25, 25,225)
G cutoff = 73.4112 ( 20595 G-vectors) smooth grid: ( 18, 18,160)
Since I have six atom layers and six vacuume layers, it seems that the code translated the FFT grids along the z direction about eleventh of the z-axis length to judge whether the grid (nr3) can coincide with itself. It failed due to 225 can't be divided by 11. I have tried to set "nr1=25, nr2=25, nr3 =220" manually (also nr3=231 and 242), but these FFT grids were not allowed.
A solution to this may be to change the FFT library which allows factor of 11, and recompile the PWSCF code. Another solution to this problem may be to reduce or increase the vacuum layers so that the FFT grids are appropriate for the symmetric operations. But since I want to see the dependence of relaxations of surface layers on the slab thickness, I have to fix the vacuum thickness for different slabs. Is there anyway to get symmetric relaxations of different even-layer slabs? For example, is it possible to shift the FFT grid at the setup so that it coincides with symmetry of different slabs?
Best regards,
Lucky
Dear Lucky:
the non-symmetric part of the relaxation is very small and possibly not very meaningful. Nevertheless, I agree that if reflection were an exact symmetry of your system, this asymmetry should vanish. One possible explanation could be that reflection symmetry is (slightly) violated by your numerical setup, e.g. the real-space fft grid could be not symmetryc.?/DIV>
Stefano
On Oct 11, 2006, at 3:14 AM, lucky8121 wrote:
Dear users,
I am a new user of PWSCF. I am doing the structure relaxation of metal slabs.
The relaxations (interlayer spacings) on the two sides of the odd-layer slab are symmetric. However, I met problem with the relaxation of even-layer slabs.
The initial positions of the Cu(111) slab were:
Cu ???0.000000000 ?0.577350269190?-1.224744871392
Cu ???0.000000000 ?0.000000000000?-0.408248290464
Cu ???0.500000000 ?0.288675134595 ?0.408248290464
Cu ???0.000000000 ?0.577350269190 ?1.224744871392
After relaxation (forc_conv_thr = 4.0D-4), the final positions are:
Cu ???0.000000000 ?0.577350269?-1.207031882
Cu ???0.000000000 ?0.000000000?-0.404842681
Cu ???0.500000000 ?0.288675135 ?0.404671325
Cu ???0.000000000 ?0.577350269 ?1.207203237
As you can see, the relaxations on the two sides of the slab are different. The same case occured whether I set "nosym" as ".true." or ".false.". This is strange. I wonder whether someone else has met the same problem and how he or she solved it. Thanks.
By the way, I have used a vacuum of 12.7 angstrom.
Best regards
lucky
lucky8121 at sohu.com
2006-10-11
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Stefano Baroni - SISSA?&?DEMOCRITOS National Simulation Center - Trieste
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