Exercise : Determine the 27Al Cq 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 !
We suggest you to use vesta for generation the POSCAR file from the AlF3.cif file. In the standard export procedure, the POSCAR file is generated with the conventional unit cell (non primitive R-cell with 24 atoms inside). Ask VESTA to reduce to unit cell to the primitive one. You will then have only 8 atoms in the POSCAR file.
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 Cq is 0.21 MHz. The nuclear quadrupolar momentum used to transform EFG in Cq 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.
- EDIFFG = -2E-2
POTIM = 0.1
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 NLSPLINE = .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
automatic mesh 0 Auto 20
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