NMR
***

Warning:

  The calculations of NMR shielding with BAND has not been thoroughly
  tested and the results might be unreliable. One should be extra
  careful when running NMR calculation, and validate the results by
  using different super-cells and different technical parameters.

With the NMR option the *shielding tensor* is calculated. There are
two methods implemented: the super cell method and the single-dipole
method.

1. The super cell method is according to the implementation by
   Skachkov *et al.* [1] The symmetry will be automatically disabled.
   The unit cell should not be chosen too small.

2. The other method is the single-dipole method. In principle one can
   now use the primitive cell [2]. In practice also this method needs
   to be converged with super cell size. However, depending on the
   system the required super cell may be much smaller. At a given
   super cell size this method is more expensive than the super cell
   method.

   NMR
      Enabled Yes/No
      SuperCell Yes/No
   End

"NMR"
   Type:
      Block

   Description:
      Options for the calculations of the NMR shielding tensor.

   "Enabled"
      Type:
         Bool

      Default value:
         No

      Description:
         Compute NMR shielding.

   "SuperCell"
      Type:
         Bool

      Default value:
         Yes

      Description:
         This is the switch between the two methods, either the super
         cell (true), or the single-dipole method (false)

This option honors the "SelectedAtoms" key, in which case only the NMR
properties will be calculated for the selected atoms only.

[1] D. Skachkov, M. Krykunov, E. Kadantsev, and T. Ziegler,  *The
    Calculation of NMR Chemical Shifts in Periodic Systems Based on
    Gauge Including Atomic Orbitals and Density Functional Theory.*
    Journal of Chemical Theory and Computation  6, 1650 (2010).

[2] D. Skachkov, M. Krykunov, and T. Ziegler,  *An improved scheme for
    the calculation of NMR chemical shifts in periodic systems based
    on gauge including atomic orbitals and density functional theory*,
    Canadian Journal of Chemistry 89, 1150 (2011).
