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# Alpha-SiO2

Exercise : Determine the 17O Cq value and the Si and O schieldings.

α-SiO2 crystallizes in the trigonal P312 space group.

a = b = 4.604 Å; c = 5.207 Å

α = β = 90°; γ = 120°

The unit cell contains two independent atoms (1 Si and 1 O) with 3 formula units (SiO2) per unit cell (Z=3). SiO4 tetrahedron units are linked together by corner sharing.

In this exercise one wants first to calculate the EFG tensor components of 17O. This is very fast task calculated at the end of the first SCF calculation (ground state property). The experimental values for the Cq is 5.19 MHz. The nuclear quadrupolar momentum used to transform EFG in Cq is Q = 2.55 10-30 m2 (see the paper of Profeta et al. (http://pubs.acs.org/doi/abs/10.1021/ja027124r) (Silicon has a 1/2 nuclear spin, so Q is zero)

In a second step one wants to calculate the shielding parameters for Si and O. 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 Profeta et al.

``` SYSTEM      = Si O2
GGA          = PE
ISTART       = 1
ICHARG       = 0
INIWAV       = 1
LREAL        =  AUTO
ISYM         = 2
ISPIN        = 1
```

Ionic minimisation

``` NSW          = 0
ISIF         = 2
IBRION       = 2
```
1. EDIFFG = -2E-2
``` POTIM        = 0.1
```

Electronic minimisation

``` IALGO        = 38
LWAVE        = .TRUE.
EMIN         =   -20.0
EMAX         =   10.0
NEDOS        = 1601
```

EFG Calculation

``` LEFG         = .TRUE.
```

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
NLSPLINE     = .TRUE.  # only needed if LREAL is NOT set.
LREAL        = A          # helps for speed for large systems, not needed
NBANDS       = 30       # to safe memory, ??? = NELECT/2
```

```automatic mesh
0
Auto
20
```
```O2 Si1
1.00000000000000
4.6040000915999997    0.0000000000000000    0.0000000000000000
-2.3020000457999998    3.9871810383000001    0.0000000000000000
0.0000000000000000    0.0000000000000000    5.2069997787000002
Si   O
3     6
Direct
0.4436617824484789 -0.0000000000000000  0.3333333429999996
-0.0000000000000000  0.4436617824484789  0.6666666870000029
0.5563382175515210  0.5563382175515210 -0.0000000000000000
0.3926661416221499  0.3062177364999842  0.2428214976299141
0.6937822635000156  0.0864484051221655  0.5761548406299137
0.9135515948778347  0.6073338583778505  0.9094881546299145
0.3062177364999842  0.3926661416221499  0.7571785323700884
0.0864484051221655  0.6937822635000156  0.4238451593700863
0.6073338583778505  0.9135515948778347  0.0905118383700884
```