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# Alpha-SiO2

* Exercise :* Determine the

^{17}O C

_{q}value and the Si and O schieldings.

α-SiO_{2} crystallizes in the trigonal P3_{1}2 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 (SiO_{2}) per unit cell (Z=3). SiO_{4} tetrahedron units are linked together by corner sharing.

In this exercise one wants first to calculate the EFG tensor components of ^{17}O.
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 5.19 MHz. The nuclear quadrupolar momentum used to transform EFG in C_{q} is Q = 2.55 10^{-30} m^{2}
(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

- 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 = 0.0 25.5

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

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