Nuclear Quadrupole Interaction (EFG)
************************************

   EFG
      Enabled Yes/No
   End

"EFG"
   Type:
      Block

   Description:
      The electronic charge density causes an electric field, and the
      gradient of this field couples with the nuclear quadrupole
      moment, that some (non-spherical) nuclei have and can be
      measured by several spectroscopic techniques. The EFG tensor is
      the second derivative of the Coulomb potential at the nuclei.
      For each atom it is a 3x3 symmetric and traceless matrix.
      Diagonalization of this matrix gives three eigenvalues, which
      are usually ordered by their decreasing absolute size and
      denoted as V_{xx}, V_{yy}, V_{zz}. The result is summarized by
      the largest eigenvalue and the asymmetry parameter.

   "Enabled"
      Type:
         Bool

      Default value:
         No

      GUI name:
         EFG (electric field gradient): Calculate

      Description:
         Compute the EFG tensor (for nuclear quadrupole interaction).

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