[SIESTA-L] lattice constant puzzle

Tue, 08 Mar 2011 14:29:04 -0800 

Dear Siesters,

I am working with b-cristobalite, a silica polymorph. I am puzzled by two
different equilibrium lattice constant i obtain using a 2-atom primitive
cell (7.14A) and a 24-atom one complete unit cell (7.57A). Maybe I doing
something fundamentally wrong, appreciate if anyone can point out the error.

Here's my fdf for 2atom version:
SystemName          b-cristobalite
SystemLabel         bcris_7.14A
NumberOfAtoms       2
NumberOfSpecies     2
%block ChemicalSpeciesLabel
 1  14  Si.gga
 2   8   O.gga
%endblock ChemicalSpeciesLabel
LatticeConstant    7.14 Ang
%block LatticeParameters
  1.0  1.0  1.0  90.  90.  90.
%endblock LatticeParameters
BandLinesScale  pi/a
%block BandLines
1   1.5  1.5  0.0   K        # Begin at K
38  0.0  0.0  0.0   \Gamma   # 38 points from K to Gamma
36  0.0  2.0  0.0   X        # 36 points from Gamma to X
18  1.0  2.0  0.0   W        # 18 points from X to W
26  1.0  1.0  1.0   L        # 26 points from W to L
31  0.0  0.0  0.0   \Gamma   # 31 points from L to Gamma
%endblock BandLines
AtomicCoordinatesFormat  Fractional
%block AtomicCoordinatesAndAtomicSpecies
    0.000    0.000    0.000     1  Si
    0.125    0.125    0.125     2  O
%endblock AtomicCoordinatesAndAtomicSpecies
%block kgrid_Monkhorst_Pack
   4  0  0  0.5
   0  4  0  0.5
   0  0  4  0.5
%endblock kgrid_Monkhorst_Pack
XC.functional           GGA
XC.authors              PBE
PAO.BasisSize     DZP
PAO.EnergyShift   300 meV
MeshCutoff        100. Ry

and for the 24 atom version, I only change the add the extra coordinates:
AtomicCoordinatesFormat  Fractional
%block AtomicCoordinatesAndAtomicSpecies
0.125 0.125 0.125 2 O1 1
0.125 0.625 0.625 2 O2 2
0.625 0.625 0.125 2 O3 3
0.625 0.125 0.625 2 O4 4
0.125 0.875 0.875 2 O5 5
0.125 0.375 0.375 2 O6 6
0.625 0.375 0.875 2 O7 7
0.625 0.875 0.375 2 O8 8
0.875 0.125 0.875 2 O9 9
0.875 0.625 0.375 2 O10 10
0.375 0.625 0.875 2 O11 11
0.375 0.125 0.375 2 O12 12
0.875 0.875 0.125 2 O13 13
0.875 0.375 0.625 2 O14 14
0.375 0.375 0.125 2 O15 15
0.375 0.875 0.625 2 O16 16
0 0 0 1 Si1 17
0 0.5 0.5 1 Si2 18
0.5 0.5 0 1 Si3 19
0.5 0 0.5 1 Si4 20
0.25 0.25 0.25 1 Si5 21
0.25 0.75 0.75 1 Si6 22
0.75 0.75 0.25 1 Si7 23
0.75 0.25 0.75 1 Si8 24
%endblock AtomicCoordinatesAndAtomicSpecies

Thank you for your time,
Prat

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