Alpha-SiO2

Exercise : Determine the ¹⁷O C_q 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 C_q is 5.19 MHz. The nuclear quadrupolar momentum used to transform EFG in C_q 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.

• INCAR

 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.
 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 

• KPOINTS

automatic mesh
0
Auto
20

• POSCAR

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

Download

SiO2_NMR.tgz

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