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<pre wrap="">Hello Quantum Espresso users,
I have a question about the disagreement in vc-relax results between
orthogonal and non-orthogonal cell. I am working on carbon structure to
get the basic properties such as energy and lattice constant using
'C.pbe-rrkjus.UPF' pseudo poential. The diamond calculations using
orthogonal (ibrav = 1) and non-orthogonal operations (ibrav = 2) agree
each other, but FCC, BCC and HCP do not.
For example, the input script of FCC variable cell relaxation using
ibrav = 2 (non-orthogonal structure) are attached below.
&control
pseudo_dir='../'
calculation='vc-relax'
tstress=.true.
/
&system
ibrav= 2, celldm(1) = 6.04709, nat= 1, ntyp= 1,
ecutwfc = 50,
ecutrho = 400,
/
&electrons
/
&ions
/
&cell
/
ATOMIC_SPECIES
C 12.0106 C.pbe-rrkjus.UPF
ATOMIC_POSITIONS crystal
C 0 0 0
K_POINTS automatic
7 7 7 1 1 1
The results using the script is listed below. (Energy and lattice
constant is converted to eV and Å, receptively.)
******** C (FCC-min) *********
Quantum espresso, C.pbe-rrkjus.UPF
k-point: 7, energy cutoff (Rad): 50
Total energy(eV); -148.951
Energy/atom(eV); -148.951
Pressure(Bar): 0.75
lattice const(Å): 3.50449
Stress tensor(Kbar)
0.750 -0.000 0.000
-0.000 0.750 -0.000
0.000 -0.000 0.750
Next input and output using ibrav = 1 (orthogonal structure) are
attached below.
&control
pseudo_dir='../'
calculation='vc-relax'
tstress=.true.
/
&system
ibrav= 1, celldm(1) = 6.04709, nat= 4, ntyp= 1,
ecutwfc = 50,
ecutrho = 400,
/
&electrons
/
&ions
/
&cell
/
ATOMIC_SPECIES
C 12.0106 C.pbe-rrkjus.UPF
ATOMIC_POSITIONS crystal
C 0 0 0
C 0.5 0.5 0
C 0 0.5 0.5
C 0.5 0 0.5
K_POINTS automatic
7 7 7 1 1 1
******** C (FCC) *********
Quantum espresso, C.pbe-rrkjus.UPF
k-point: 7, energy cutoff (Rad): 50
Total energy(eV); -601.511
Energy/atom(eV); -150.378
Pressure(Bar): -0.74
lattice const(Å): 3.09758
Stress tensor(Kbar)
-0.740 0.000 0.000
0.000 -0.740 0.000
0.000 0.000 -0.740
As seen, the energy of orthogonal structure is 2 eV lower than the
non-orthogonal structure. Also, the difference of lattice constant is
~0.4 Å.
I checked the initial and final crystal structure using Xcrysden, and
both look fine.
Could you please tell me what I am missing here?
As a reference, I attached the input scripts and output log files in
this email.
Thank you very much,
Sungkwang Mun
Postdoctoral Associate
Center for Advanced Vehicular Systems
Mississippi State University
</pre>
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