Alpha-AlF3: Difference between revisions

Revision as of 18:44, 31 August 2016

Exercise : Determine the ²⁷Al C_q value and the Al and F shieldings.

α-AlF₃ crystallizes in the trigonal R -3c space group.

a = b = 4.9305 Å; c = 12.4462 Å

α = β = 90°; γ = 120°

The unit cell contains two independent atoms (1 Al and 1 F) with 6 formula units (AlF₃) per unit cell (Z=6). AlF₆ octahedron units are linked together by corner sharing. As the conventional unit cell is non-primitive, the primitive rhombohedral one is used for the calculation. It saves a lot of computational time !

In this exercise one wants first to calculate the EFG tensor components of ²⁷Al. This is very fast task calculated at the end of the first SCF calculation (ground state property). The experimental values for the C_q is 0.21 MHz. The nuclear quadrupolar momentum used to transform EFG in C_q is Q = 14.66 10^-30 m² (see the paper of Sadoc et al. (http://www.sciencedirect.com/science/article/pii/S0926204014000022) (Flurine has a 1/2 nuclear spin, so Q is zero)

In a second step one wants to calculate the shielding parameters for Al and F. This is done using the linear response using the GIPAW formalism. As the calculation is quite time consuming, only very few k-points and small ENCUT are used with standard PAW data sets. The calculated shielding tensors components can be compared to the ones obtained by Sadoc et al.

INCAR

  SYSTEM       =  Al F3

  GGA          = PE
  ISTART       = 1
  ICHARG       = 0
  INIWAV       = 1
  LREAL        =  AUTO
  ISYM         = 2
  ISPIN        = 1

Ionic minimisation
  NSW          = 0
  ISIF         = 2
  IBRION       = 2
#  EDIFFG       = -2E-2
  POTIM        = 0.1

Electronic minimisation
  IALGO        = 38

  LWAVE        = .TRUE.

  EMIN         =   -20.0
  EMAX         =   10.0
  NEDOS        = 1601

EFG Calculation
  LEFG         = .TRUE.
  QUAD_EFG     = 146.6 0.0

Chemical Shift
  PREC         = Normal    # nice
  ENCUT        = 400.0      # typically higher cutoffs than usual are needed
  ISMEAR = 0; SIGMA= 0.1 # no fancy smearings, SIGMA sufficiently small
  EDIFF        = 1E-9      # you'd need much smaller EDIFFs.

  LCHIMAG      = .TRUE.   # to switch on linear response for chemical shifts
  DQ           = 0.001         # often the default is sufficient
  ICHIBARE     = 1       # often the default is sufficient
  LNMR_SYM_RED = .TRUE. # be on the safe side
  NSLPLINE     = .TRUE.  # only needed if LREAL is NOT set.
  LREAL        = A          # helps for speed for large systems, not needed
  NBANDS       = 25       # to safe memory, ??? = NELECT/2

KPOINTS

automatic mesh
0
Auto
20

POSCAR

Al1 F3
1.0
        4.9305000305         0.0000000000         0.0000000000
        2.4652500153         4.2699382798         0.0000000000
        2.4652650832         1.4233214594         4.1486879977
   Al    F
    2    6
Direct
     0.000000000         0.500000000         0.000000000
     0.500000000         0.000000000         0.500000000
     0.177499995         0.250000000         0.750000000
     0.822499990         0.750000000         0.250000000
     0.677500010         0.322499990         0.250000000
     0.322499990         0.677500010         0.750000000
     0.250000000         0.177499995         0.250000000
     0.750000000         0.822499990         0.750000000

Download

Ni111clean_400eV.tgz

To the list of examples or to the main page

@@ Line 1: / Line 1: @@
-''<u>Exercise :</u>'' Determine the <sup>27</sup>Al C<sub>q</sub> value and the Al and F schieldings.
+''<u>Exercise :</u>'' Determine the <sup>27</sup>Al C<sub>q</sub> value and the Al and F shieldings.
+&alpha;-AlF<sub>3</sub> crystallizes in the trigonal R -3c space group.
+a = b = 4.9305 &Aring;; c = 12.4462 &Aring;
+&alpha; = &beta; = 90°; &gamma; = 120°
+The unit cell contains two independent atoms (1 Al and 1 F)  with 6 formula units (AlF<sub>3</sub>) per unit cell (Z=6). AlF<sub>6</sub> octahedron units are linked together by corner sharing.
+As the conventional unit cell is non-primitive, the primitive rhombohedral one is used for the calculation. It saves a lot of computational time !
+In this exercise one wants first to calculate the EFG tensor components of <sup>27</sup>Al.
+This is very fast task calculated at the end of the first SCF calculation (ground state property).
+The experimental values for the C<sub>q</sub> is 0.21 MHz. The nuclear quadrupolar momentum used to transform EFG in C<sub>q</sub> is Q = 14.66 10<sup>-30</sup> m<sup>2</sup>
+(see the paper of Sadoc ''et al.'' (http://www.sciencedirect.com/science/article/pii/S0926204014000022)
+(Flurine has a 1/2 nuclear spin, so Q is zero)
+In a second step one wants to calculate the shielding parameters for Al and F. This is done using the linear response using the GIPAW formalism.
+As the calculation is quite time consuming, only very few k-points and small ENCUT are used with standard PAW data sets.
+The calculated shielding tensors components can be compared to the ones obtained by Sadoc ''et al.''
 *INCAR