Hey Prat,
b-cristobalite lattice parameter is 7.16 experimentally and it's stable
at high temperature (above ~1770K).
I can send you the cif file I usually use for this system.
Eric.
On 03/08/2011 05:28 PM, Prathibha Ramaprasad wrote:
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
0.1250.1250.1252O11
0.1250.6250.6252O22
0.6250.6250.1252O33
0.6250.1250.6252O44
0.1250.8750.8752O55
0.1250.3750.3752O66
0.6250.3750.8752O77
0.6250.8750.3752O88
0.8750.1250.8752O99
0.8750.6250.3752O1010
0.3750.6250.8752O1111
0.3750.1250.3752O1212
0.8750.8750.1252O1313
0.8750.3750.6252O1414
0.3750.3750.1252O1515
0.3750.8750.6252O1616
0001Si117
00.50.51Si218
0.50.501Si319
0.500.51Si420
0.250.250.251Si521
0.250.750.751Si622
0.750.750.251Si723
0.750.250.751Si824
%endblock AtomicCoordinatesAndAtomicSpecies
Thank you for your time,
Prat
--
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/ --- Mahatma Gandhi ---
Dr. Éric Germaneau <mailto:[email protected]>
College of Physical Sciences
Graduate University of Chinese Academy of Sciences
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Beijing 100049
China
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